EXP per unit killed

[Seravy]

You get XP for dead combat units at the end of combat. Combat summons are dead combat units, they don't magically vanish when they die. You even get fame for them.

The AI doesn't really care about EXP anyway - most of their units only fight one battle against a human and get killed. (and against other AI both don't have the EXP)
If absolutely necessary I would prefer to fix this by implementing an AI EXP multiplier based on difficulty, as EXP is one of the things the AI is bad at gaining and using.

I'll read the rest and reply later.

[zitro1987]

With doom-stack making in mind:

1 EXP : Magic Spirit, Skeletons, phantom Warrior   (doom-stacking with these will not happen, experience expected minimal)
2 EXP : Hell Hounds, Guardian Spirit, Zombies, Fire Elemental    (moved a couple of combat summons up for being inferior to 3xp units)
3 EXP : Nagas, War Bears, Sprites                 (moved sprites up to encourage stacking with bears, and sprites are not too menacing in wizard-to-wizard battles)
4 EXP : Ghouls, Phantom Beasts, Earth Elementals     (ghouls ended up alone, passed them a couple of combat summons)
5 EXP : Werewolves, Gargoyles, Fire Giants, Giant Spiders      (probably fine as is)    
6 EXP : Chimeras, Unicorns, Cockatrices, Basilisk, Demons, Water Elementals              (basilisk need unicorns badly, you forgot water elemental)
8 EXP : Chaos Spawn, Shadow Demons, Angel, Night Stalkers, Air Elemental    (air elemental may be fine with 8pts because they are tough as neutrals and often in nodes ... here we have toughest uncommons and weakest rares)          
10 EXP : Hydra, Wraiths, Stone Giants   (moved stone giants to 10pts ... hydras deserve more, but I wouldn't want doom-stacking with fast chaos units, so staying at 10)
12 EXP : Doom Bat, Efreet, Storm Giant, Gorgons   (moved gorgons to 12pt, would pair really well with anything here if multi-color, and they are not THAT good for 15xp)
16 EXP : Death Knights, Djinn, Demon Lord? (16 + 3*6 = 34 total exp with demons?) 
20 EXP : Arch Angel, Behemoth   (you forgot behemoth, may be a good 20xp unit)    
25 EXP : Sky Drake, Colossus,  Great Drake, Great Wyrm

*I see the concern of EXP farming that Nelphine raised. However, I prefer to suggest moderate EXP gains to consider neutral fights - let's just rate them as closely to general difficulty rather than artificially high. Earth Elementals are typically easy to handle. I wish they had 2mv. Thus 4xp seems fair, barely below tough units like fire giants.

[Nelphine]

Even ignoring the AI not getting the same xp as the human, why should fighting a nature wizard give me far more experience than fighting any other wizard?
Why is earth elemental SO MUCH better than flame strike?? Again, if you're going to keep combat summon experience in (even if its due to coding reasons), they should be dropped to virtually zero. And I'd include centaurs and catapults in this, giving them an arbitrarily low experience (like 1) - the AI almost never uses either unit outside of the combat summon anyway, so it wouldn't actually change anything except for the combat summon.

That being said, getting experience from the enemy casting spells does make sense; I'd be perfectly happy giving out 1 experience for every 10 casting skill the enemy wizard has. (This would also make it less mandatory to get your best heroes super early so they can reach demi god status before you fight the third AI - you'd be able to actually get experience from enemy AI, so you could actually level up a late game hero.)

Getting heroes up levels for the human is absolutely huge. The way it is now, saving a sad nature AI with one city just to farm earth elemental experience is a real thing.

[Seravy]

Summoned creatures give you presence in battle. They prolong combat, benefit from your city walls, buffs, etc. Flame Strike does none of these.
Neutrals giving half experience is what compensates for the experience value of magic so we already have that, it just does not scale on the amount of spells used (but stronger, higher EXP value armies usually correlate with stronger spells and longer combat. Even in the case of the "lone swordsmen" - you either kill it fast and don't get much EXP or the AI has to summon things which increases the EXP you are gaining but gives them more time to keep casting.)

Will mark ideas I find work considering bold.

Catapults and Centaurs are normal units - we can't change their costs for experience reasons.

I notice I missed Skeletons on my list, they are in the 30 cost group currently.

I still disagree with 2 EXP zombies and Fire Elementals.

I'm unsure if I want to stack Sprites with Bears and Nagas instead of Ghouls - that might actually be a good idea. Unfortunately it might dilute the strategic combat value of naga stacks, making it harder for the AI to actually attack anything with the stack. (this is not an issue with bears - 9 water walking bears won't happen anyway)
However sprites would also slow down the movement speed of the stack. So there are upsides (harder to defend against a mixed naga/sprite stack, especially in the capital) but downsides as well (slower stack with weaker total strategic strength so easier to intimidate and prevent from attacking, and less effective at attacking neutral targets)

Ghouls being alone would probably be fine - they aren't a good combo with sprites anyway, as sprites are less durable and do not make undead as well as diluting overland strength. They also won't stack up unless all wraithformed which is not likely to mean 9 or more units.

Moving Sprites into their own category (2, 3 or 4 exp as desired, but a different cost than 40/70/100) is also an option.

I'm against reducing the EXP value of combat summons in general. Demon might be the exception - I'm unsure if it's really good enough to be worth 6 EXP when it's barely better than a Fire Elemental, albeit Flying is very good at providing combat presence so maybe worth it anyway. (also the spells it casts or the touch attacks it gains are powerful)

Great Lizard I very much rather pair with Angel - Neither unicorns nor lizards fly and unicorns are extremely unlikely to even get water walking on them because lizards are plain stronger and thus higher priority to target. Angels on the other hand fly already so they work. Having a regenerating ground creature with a nonregenerating flier (which is harder to attack for the human) seems better anyway.

Raising all of Shadow Demons, Lizards, Chaos Spawn and Angel to 8 I guess isn't a bad idea albeit I'm hesitant to make the slow Chaos Spawns worth more but 7 or 8 is not a big difference.
However I'm also unsure if I want Shadow Demons to stack with Night Stalkers (as both are Death but one does not regenerate. Pure Shadow Demon stacks are scarier.) so in this case Nigh Stalker would need to have a different cost I think.

We probably should make Earth Elemental stay at 8 EXP but have a cost of 239 so it does not accidentally stack with the above (if raised) and slow the stack to 1 moves.

We could make Hydras 11 EXP to ensure they don't stack with anything. Wraiths are fast - it's probably a good candidate to mix with the 12 EXP strong and fast chaos units albeit it's not as hard to defeat as those. They would likely perform well near Storm Giants and Stone Giants as well.

I guess moving Stone Giants down is ok - it's unlikely to have them water walk and they won't mix with Storm Giants (one or the other would take priority and get all the water walk buffs).

I disagree moving Gorgons down - they are slower than efreet and doom bats, and not ranged like the giants, so they would make the stack weaker by being the only slow melee unit among lots of ranged or fast units.

I'm not moving Demon Lord, Great Drake or Great Wyrm - those should be the same group for sure as Archangel. I'm also strongly against adding ANYTHING to the Sky Drake/Colossus group as those creatures are very self-sufficient and formidable if not mixed with others. Adding something that's not Magic Immune (or not ranged and illusion immune in case of Colossus) would just make the stack easier to destroy.

Everything in the 20 EXP group moves 4 - Behemoths would be poor performers there. It might be good to make them 18 EXP so they stay alone if the wizard has no better creatures, allowing powerful all regenerating stacks.

Farming EXP from combat summons, well interesting idea but if the wizard can't cast anything that can harm my hero, I can just kill all their other units instead anyway. And it takes a hell of a lot powerful hero to not get hurt by an Earth Elemental. (farming Air Elementals is pure masochism though. I won't worry about that becoming popular)

[Nelphine]

First: no way is an earth elemental worth more than the 'slow moving chaos spawn'.

Second: getting a true sight hero capable of healing itself either via magic or attacks (vampiric or battle mage with life like Torin) or just having ranged attacks is EASY. Farming elementals of any kind is simple. There's a reason why earth elementals are awful when defending nodes.

Third: of courses summons do things that flame strike doesnt. But there's no WAY those things are so much better than flame strike that you deserve more XP because the AI cast that spell. Flame strike does tons of things earth elementals don't. (Like kill ranged units...) The spells are balanced without this major downside of combat summons giving more xp.

Centaurs and catapults, I would put an exception into the experience table that overwrites how much they're worth. In other words, calculate experience for all normal units, then add in an extra modifier for centaurs and catapults that effectively cancels it out.

Fire elementals are crap. The only reason they exist is for very early game conquering of neutrals. You've already stated that AI XP doesn't matter, which means the only time fire elemental XP matters is against the human. If the ai used it properly super early, the ai wins so the XP value doesn't matter. If the AI uses it any other time, it's like giving free xp to the human, so it should be low. And nodes/lairs are the same. Either the human can't beat them so XP doesn't matter, or they're free XP so it should be low.

Zombies: 
In my last 30 games I have not faced zombie mastery. But I've killed lots of zombies. Easily. Because they're speed 1. A Bowman kills them. Either they're a combat summon, in which case they're already balanced against other combat spells WITHOUT giving extra XP, or they're a lair creature in which case the reward is the treasure. Either way, XP 1. Zombie mastery happens? Well, it's rare and it makes crappy units (ghouls make far far better units) so it's already underpowered. Let's not give out free XP at the same time.

Ghouls and naga should go together. They're both amazing. Sprites shouldn't be joined with either, they're too weak. (Also yes, I get attacked with stacks of 7+ water walking war bears on master and lunatic. This is a real thing.)

Angels go far better with wraiths than lizards (overland speed!! The biggest reason the AI still is bad at offense is because they're slow! Let's let them make use of that speed 5+) Angels are also rare and actually decently strong in lairs, so certainly are worth more comments than a lizard. Unicorns give a bigger boost to resistance than unicorns. Why would you keep angels with lizards? If unicorns din get water walking enough, raise the unicorn water walking priority.

[zitro1987]

Earth elemental needs to be its own category, no way it's worth 8. Phantom beasts also high.

[Seravy]

But there's no WAY those things are so much better than flame strike that you deserve more XP because the AI cast that spell.

I disagree here. Combat summons enable you to cast more spells. Attack my spearmen. I cast Flame Strike, then battle ends. Or I cast Air Elemental, then Flame Strike 5 times, then battle ends. Guess which should be worth more EXP. (Yes, Flame Strike granting EXP would be better but I can't do that. And then people would start farming EXP of holy armors and similar then retreat because, hey, they already got the EXP, no need to win the battle.)

First: no way is an earth elemental worth more than the 'slow moving chaos spawn'.

Well the Earth Elemental allows me to cast 3 more Flame Strikes (or let Call Lightning hit 12 more times), the Chaos Spawn...technically it does too but it needs to be already in the battle so I'd need to prepare it in advance and the enemy would be able to see and not attack it. So the Earth Elementals do it better. Additionally, the Earth Elemental I can summon right next to the weakest enemy unit and kill them (the AI can too, albeit they aren't very good at it) while the Chaos Spawn has no way to avoid having to reach the enemy using its own, low movement.

I've seen Hadriex lose major cities with large garrisons to an AI summoning an Earth Elemental every turn with Call Lightning in play - and those battles most definitely should not be worth crappy experience.

I did think about the amount of 8 being right or not, but...yeah, this is a 30 health, 25 attack power unit. Admittedly it's weak to ranged attacks, but ammo will eventually run out and these are quite tanky even with the horrible 4 armor. I think 8 is fair.

Fire Elemental, I guess I can lower that. Not that 3 EXP is a relevant amount - takes 10 of these for a single barracks worth of experience. Early AI can afford making one or two at best in a battle. So I don't really see a need for it - getting an additional point of experience for being able to overcome weapon immunity isn't unreasonable.

Not going to lower zombies. 18 health is extremely tanky and buys tons of time for casting spells (or forces the enemy to spend a very expensive late game spell like annihilate on it) making it worth a lot.

One more thing, you should consider the "other side". Yes, AI experience is unimportant, except for one specific case : where the AI stack already won against the human and proved that it won't die on their first use. So rewarding the AI for having a stack that can not only win the battle but ALSO kill all the combat summons the player tries to use to stall for time makes sense. This isn't the most important factor ever, but it's there.

Okay so let's try to make the new list. (why are skeletons 1 btw? They are not much weaker than a hell hound especially now with 7 figures. Anyway, anything I'm still unsure of, I'll list on both possible locations.)

30 / 1 = Magic Spirit, (Skeletons?), Phantom Warriors
ok, the phantom warriors are the easiest thing to kill ever. Though they cause painful losses if the player is not careful and they are summoned in striking distance, that by itself shouldn't worth exp
40 / 2 = Hell Hounds, Guardian Spirits, (Skeletons?)
There is no way Hell Hounds are worth the same as bears.
69 / 3 = Sprites
70 / 3 = Phantom Beasts, Zombies, War Bears, (Nagas?)
The phantom beasts actually do a crappy job at prolonging battles with zero armor (even zombies are better at it). They do nasty damage but I guess the 5 EXP wasn't really justified for them.
Speaking of Zombies, Zombie Mastery is an insanely overpowered spell but I know some people disagree with that. I guess I can agree with sprites and bears working well together but only in normal combat.
Sprites have much worse strategic combat value than either bears or nagas. If we go with the assumption that the AI does have 9 water walking bear stacks, sprites should not be included - if it lacks the needed amount of bears, it will fill the gap with lower value units (sprites) anyway.
I checked and the AI does pick both Sprites and War Bears as starting spells, so this is a fairly likely scenario we should properly handle. On the other hand the AI never picks Sprites along with either Ghouls or Nagas so we don't need to worry about those pairings appearing in most games (yes they might find the spell eventually but by then they'll be outdated). Zombies will almost never gain water walking and don't appear early on the overland map so we can ignore them for stacks.
100 / 4 = Ghouls, (Nagas?)
Unsure about mixing these - it lowers the potential for using darkness, create undead or movement speed overland, with the only gain being the ability to use ranged attacks - but that is a fairly big gain of its own. The AI can pick these two as starting spells plus wraith form for the ghouls so this decision is important. I feel mixing Nagas with water walking Bears might be better.
130 / 5 = Gargoyles, Werewolves, Giant Spiders, (Fire Giants)
210 / 7 = (Fire Giants), Water Elemental, Cockatrices, Chimeras, Unicorns
Honestly, I don't think cockatrices really belong here and might be better at 5 exp (they are a joke as soon as you have any 14 resistance unit) but with the AI being good at putting focus magic on them, grouping them with stronger creatures makes sense. Also being the best strategic value uncommon create in the game deserves a high ranking. I have no real arguments against a higher value fire giant - they are very similar to water elementals anyway. However I do not think mixing them with chimeras is a good idea - chimeras are way stronger both in actual combat and strategic. Meanwhile Gargoyles most definitely can use ranged support from a flying or water walking giant. The AI will research both the Giant and the Chimeras if they show up in the correct order, so this is a mixing situation that's relevant. (albeit the giants are not that likely to fly or waterwalk.)
238 / 8 = Earth Elemental, (Chaos Spawn)
239 / 8 = Air Elemental, Night Stalker
I disagree with reducing the elementals, but combining them with other units is bad so they should be their own group.  I wouldn't mind reducing Night Stalker but I like having the two invisible creatures together. Not that wraithformed stalkers stacking with undead air elementals and move while hidden in a stack of 9 is a likely outcome but the possibility should exist.
240 / 8 = Shadow Demons, Great Lizard, (Chaos Spawn), (Angel)
I disagree with putting unicorns here. Having water walking lizards and unicorns at the same time just won't happen, especially not in meaningful quantities. (also Cockatrices need the resistance just as much as lizards. So does giants and water elementals and even chimera))
300 / 10 = Hydra
I see no meaningful reason why the AI should prioritize pairing Hydra with Chaos Spawn, instead of the other regenerators. If anything, the Chaos Spawn should be prioritized below those, not above. More EXP I could see but here the stackbuilding is much more important.
329/11 or 301/10 = (Stone Giant)
330/11 = Efreet, Storm Giant, (Stone Giant), (Doom Bat),(Djinn)
Unsure if mixing efreet with the stone giant (for higher strategic stack strength) or keeping it pure efreets (for better normal combat effectiveness) is the better tactic here. Storm giants fill the same role as Efreet very well so those are fine. Doom Bat is a fine contributor to an otherwise ranged stack with its 6 move and doom attack, but using it by itself for faster movement can be even better. Pure doombat stacks have a weakness to resistance targeting spells like holy word or massacre but that's acceptable I guess.
360/12 = Wraiths, (Angel), (Doom Bat)
If we want the fast stacks to happen, we should put them above other rares, not below - the AI picks the 9 highest for the stack, not any 9 with the same group.
450 = 15 EXP : Gorgons, Behemoths, (Djinn)
If we do not want the slow behemoths and gorgons to replace the units in the fast stack, we need to put them at a lower exp value than those. Considering how strong they are, that might be unrealistic, albeit Gorgons are reasonably easy to kill and would be out of place in a stack of any of the regenerating, or fast flying groups as well. Their strategic value is huge however, so prioritizing them does help the AI crack hard nodes and lairs, thus I think they should here. Djinn have bad strategic value compared to these, so I have no idea what to do with it. Pure djinn stacks are formidable in normal combat and a high exp reward for them is fair, so rating it higher is an option - but doing so would mean a lower strategic value doomstack. Rating Djinn lower would allow the AI to not use them in the doomstacks - so they can get pulled by land stacks where their wind walking can be relevant - but this would make the exp reward too low for what's the hardest to kill creature in the game. As the nature creatures are land based, it's also realistic to expect the AI to fill the missing units (as there aren't enough with water walking) using djinn so the wind walking can even speed up the doomstack occasionally. It also allows all regenerating behemoth doomstacks to form if the AI does not know gorgons, instead of mixing in djinn, but honestly the mixed stack would be scarier there.
480 = 16 EXP : (Death Knights), (Djinn)
Death Knights have better strategic strength then even the gorgons and behemoths, and are a fast flying creature so letting them form 9 stacks is the best. In fact the strength is on par with the creatures in the top categories, although normal combat performance against the human likely isn't.
550 = 18 EXP : (Death Knights), (Arch Angels), (Great Drakes)
If we decide to separate these from the others, see below...
610 = 20 EXP or 769 = 25 EXP or anything between : Demon Lord, (Archangel), Great Wyrm, (Great Drake), Colossus
The archangel and the drake can be kited, the others can't so it might be better to move them. However based on strategic force, and holy bonus, it's likely better to let them mix anyway. Moved colossus down here - mixing them with any of these is not bad and it even helps avoiding a 9 water walking colossus stack which is vulnerable to dispelling wave.
770 = 25 EXP : Sky Drake
Sky Drake must be the top creature on its own to ensure fully magic immune stacks. It's not really that good in small numbers, but in full stacks...

[Nelphine]

Oh, right, sky drakes need to be the top to get mass numbers. That's fine. Demon Lords/Archangels/whatever else you stick with them should also be 25. Death Knights with speed 5 I really wouldn't suggest mixing with anything else - pure stacks of them would be great. Of course, if you put them below, then you don't use them, so that makes them useless. I'd say put death knights with the demon lord/archangel group.

I don't think great wyrms and colossus should be with the demon lord/archangel group. While they can use the buffs, it slows the group down. Djinn on the other hand are too weak for that group, and slow down death knights.
Gorgons and Behemoths are also super hard to place, so I agree leaving them together. Also, you convinced me that wind walking Djinn with the gorgons and behemoths is a great idea.

I'm still a fan of doombats by themselves, 1 beneath angels/wraiths, but leaving all 3 together as well is ok. But those 3 are far too valuable to the AI with overland 5 or 6 to combine with anything slower. I would never let efreet mix with doombats, let alone anything slower.

Hydra don't really need to be combined with chaos spawn, but chaos spawn are very hard hitting with high hp; they're much better than earth elementals in terms of xp, once they're already in combat, which is the only time it matters. But they slow absolutely everything else down, and that's not worth combining with anything else. Hydra do exactly the same thing, slow everything down. They can't be combined with anything else. I'd be fine with both of them being in their own group. Realistically hydra are just amazing defensive units and should never be anywhere that's not a garrison. Chaos spawn really don't have a role in AI play.

Air elementals aren't worth that much experience - same as fire elementals, either you lose to them, or you defeat them easily. There's no in between because they rely on trickery. I'd put them at most at 6 (and drop nightstalkers to the same.)

Cockatrices have always been the single most dangerous doomstack I've ever seen the AI use - noting that by late game, I don't ever get threatened by AI doomstacks. I'd actually go the opposite of you, and consider combining them with great lizards and shadow demons, and chimera. Hey shadow demons! shadow demons are speed 2.. they should go with the chaos spawn, probably xp 8, above your proposed nightstalkers. We can't split focus magic cockatrices from non focus magic can we? If we could, I'd totally agree with you.

Earth elemental: You're basically using them as mini hydras. Um. I dunno. I don't think 2 earth elementals is better than a single hydra. Which means earth elemental should at most be 5. Hell, I don't think 3 are as good as a single hydra. So that makes them 3. So lets increase hydra a bit instead.

Fire giants: I'd like to toss them in with water elementals just because it's a good combo (fireball to soften up, then water elementals have lots of ranged to hurt them with)

Zombies at best should be a 2, and by themself. (If it were me I'd make a 2 row for zombies and earth elementals, but I understand that isn't going to happen :P) You can't let the ai use them in a doomstack, no matter how unlikely it is.

Hell Hounds: Ugh, yes I agree hell hounds aren't worth the same xp as a war bear. But realistically, they're both just bad beyond the very earliest stage. And water walking war bears are usually scarier than sprites. Meh.

So..

30 / 1 = Magic Spirit, Phantom Warriors
39 / 2 = Zombies
40 / 2 = Hell Hounds, Guardian Spirits, Skeletons
41 / 2 = Sprites
70 / 3 = Phantom Beasts, War Bears, Nagas
99 / 4 = Earth Elemental
100 / 4 = Ghouls
130 / 5 = Gargoyles, Werewolves, Giant Spiders
210 / 6 = Air Elemental, Night Stalker
239 / 7 = Fire Giants, Water Elemental
240 / 8 = Cockatrices, Chimeras, Unicorns, Great Lizard
270 / 9 = Shadow Demons, Chaos Spawn
330 / 11 = Efreet, Storm Giant, Stone Giant
360 / 12 = Wraiths, Angel, Doom Bat
390 / 13 = Hydra
450 / 15 = Gorgons, Behemoths, Djinn
600 / 20 = Great Wyrm, Colossus
769 / 25 = Demon Lord, Death Knight, Archangel, Great Drake
770 / 25 = Sky Drake

Edit: the 7 to 9 lines are the most controversial for me, and could be mixed all over the place, and I'd be happy.

[Seravy]

Oh, I knew there is one more use of these cost numbers but forgot what. Now I remember.
It also equals the amount the AI can raise using Life Steal or Create Undead.
So raising/lowering the value of Demon Lords, Death Knights, Wraiths, and Ghouls directly affects how many undead creatures the AI gets. This likely mostly matters on Ghouls - A stack of 9 Wraiths has enough total value to raise even the strongest creatures as undead I believe (don't remember what the multiplier is, 1 raised cost for 1 cost of create undead or less.) - but it's also quite relevant for zombies. Unless we are unhappy with the current state of the game, we should avoid changing these two.

I don't think great wyrms and colossus should be with the demon lord/archangel group. While they can use the buffs, it slows the group down.

Only the demon lords. Archangels wind walk.

If we want Doom Bat alone, it should be 361. It doesn't really benefit much from getting mixed with Wraiths, albeit it does benefit from Angels (1 resistance is nice for a 9 resistance creature.), it isn't enough to be important.

I don't think Death Knights are as good as the other very rares and worth mixing, especially not with demon lords. While the strategic strength is reasonable for that, normal combat effectiveness vs humans is a lot worse and on par with great drakes. (yes, 1 more move and even noncorporeal, but weak to resistance based spells, loss of figures, high resistance enemy units, and just generally everything at very rare level and even has lower armor.)

Cockatrices have always been the single most dangerous doomstack I've ever seen the AI use

While they are the most scary doomstack, they are also the easiest to counter - 3-4 gargoyles can kill the entire stack. I guess you don't play much mono-chaos. Lacking that, Resist Magic or Resist Elements on anything with 10 resistance and decent stats (most heroes, most dwarf units, several summons, nightmares, etc) will do. Lacking even that, heavy ranged still is enough to kill at least half of the stack.

I most definitely want the two big uncommon regenerators, shadow demons and lizards, together. Especially because they even work well with their stats to counter almost everything (1/4 figures, melee/ranged, flying+noncorporeal/3 moves, etc). Having Hydras near them is also convenient - while they will rarely end up in attacking doomstacks, if they do, they are all slow and regenerate yet have 3 entirely different combat roles.
We can't put hydra above 11 otherwise it ends up in the fast, move 4-6 stacks.

Zombies (...) You can't let the ai use them in a doomstack, no matter how unlikely it is.

0%, they won't put water walking on something that weak when they have rares and normal units to target.


One more thing I haven't yet considered. Stacking is also relevant for normal units so costs should/should not match those accordingly. I don't think we need to really care about anything below magician (120) as they can only possibly get used with/instead of commons.
Which leaves :
150 Nightblades, Jackal Riders, Halfling Magician, Rangers, War Trolls
160 Manticores, Warlocks, Elven Lord, Stag Beetle
180 Golem, Troll Magician, Griffins, War Mammoths (<-this used to be what got mixed with the chimera group)
200 Minotaurs, Nightmares, Air Ship
210 Doom Drakes (but if we move those up then this will be instead)
240 Paladin (can pair with great lizard group, nice. Movement is wasted though.)

[Seravy]

Trying to put everything together into an easy to read table :

30 / 1 =  Magic Spirit, Phantom Warriors
40 / 2 = Hell Hounds, Guardian Spirits, Skeletons
69 / 3 = Sprites
70 / 3 = Phantom Beasts, Zombies, War Bears, Nagas
100 / 4 = Ghouls
130 / 5 = Gargoyles, Werewolves, Giant Spiders, (Demon)
160 / 6 = (Demon)
210 / 7 = Fire Giants, Water Elemental, Cockatrices, Chimeras, Unicorns, [[Doom Drakes]]
237 / 8 = Earth Elemental
238 / 8 = Chaos Spawn
239 / 8 = Air Elemental, Night Stalker
240 / 8 = Shadow Demons, Great Lizard, Angel, [[Paladins]]
300 / 10 = Hydra
329 / 11 = (Stone Giant)
330 / 11 = Efreet, Storm Giant, (Stone Giant)
360 / 12 = Wraiths, Angel, Doom Bat
450 / 15 = Gorgons, Behemoths, Djinn
550 / 18 = Death Knights,  Great Drakes, (Archangel)
610 / 20 =  Demon Lord, (Archangel), Great Wyrm,  Colossus
611 / 20 = Sky Drake

Note if you put Hydra at 13 that lone water walking hydra will be the part of your doom bat stack. That's the worst possible place for it in the entire table.

Moving Death Knights from 450 to 550 is an increase of 18.9% in the amount of undead they can raise, putting them nearly equal to Demon Lords. (Considering they are actually better at it in real combat, that's fine?)

Arcahngel support would be an amazing help for the death knights and great drakes, both of which could use buffs and healing and would be unstoppable with it.
Demon Lords, Wyrms and Colossus are powerful enough without so probably need it less - but definitely could use the wind walking (which also ensures the doomstack cannot drown if dispelled.)

I definitely do not want to raise the Demon Lord to create even more undead, but merging the 18 and 20 groups or let Archangel buff both is an option. Unfortunately, in this case, the superior wyrms and colossus might get diluted by easy to kite drakes and death knights which is bad.

Sky Drake being the top creature is definitely intended, but maybe it should only be 611/20? It's actually easier to kill than the rest if you have web and/or aren't relying on spells so not sure about that extra 5 exp, albeit I don't mind it either.