Tweaked this a little while implementing it, it became no effect for the first 6 damage, then 1 point ignoring defense at 7, and 1 more for each 2 damage dealt.
What this means in practice...
To block 6 damage, you need 20 shields on average (without a + To Def bonus, for simplicity, count each + To Def as an additional 5 shields instead as that's the net effect of it above 15 shields. Invulnerability is roughly equivalent to 6 shields.)
20 shields is pretty much unlikely on anything except hero units, unless the wizard is massively stacking buffs from the Life realm. As that has already been established to be overpowered, it doesn't matter if it is affected - in fact, having a normal unit not take damage from a colossus is wrong in the first place, buffs or not.
Above that, you only need half as many additional shields to block damage as usual, because the other half of the damage will be ignoring it anyway. As you generally need to aim for higher defense than the attacker's attack to account for unlucky rolls, this still means you'll need about as many shields as the attacker's attack power, so shields won't be useless, even very high amounts of it. For example a 25 sword +3 Hit creature deals 15 damage. While 5 of those are minimal damage, to block the remaining 10, at least 33 shields or equivalent effects are needed on average rolls, but to be safe against "good" rolls, even more is useful. Meaning stacking lots of defense still has value - at least up to ~40 is useful against unbuffed very rare creatures, more against buffed ones or Great Drakes.
What this means in practice...
To block 6 damage, you need 20 shields on average (without a + To Def bonus, for simplicity, count each + To Def as an additional 5 shields instead as that's the net effect of it above 15 shields. Invulnerability is roughly equivalent to 6 shields.)
20 shields is pretty much unlikely on anything except hero units, unless the wizard is massively stacking buffs from the Life realm. As that has already been established to be overpowered, it doesn't matter if it is affected - in fact, having a normal unit not take damage from a colossus is wrong in the first place, buffs or not.
Above that, you only need half as many additional shields to block damage as usual, because the other half of the damage will be ignoring it anyway. As you generally need to aim for higher defense than the attacker's attack to account for unlucky rolls, this still means you'll need about as many shields as the attacker's attack power, so shields won't be useless, even very high amounts of it. For example a 25 sword +3 Hit creature deals 15 damage. While 5 of those are minimal damage, to block the remaining 10, at least 33 shields or equivalent effects are needed on average rolls, but to be safe against "good" rolls, even more is useful. Meaning stacking lots of defense still has value - at least up to ~40 is useful against unbuffed very rare creatures, more against buffed ones or Great Drakes.
