Resist magic adds resistance, not defense, so it has no effect on anything that does direct damage (fire bolt, fireball, fairy dust, etc). However, nothing works to stop the flat 12 damage of doom bolt except complete magic immunity. Invisibility too, assuming you're not fighting a Life, Death or Chaos wizard with the proper countermeasures. The lack of any viable defense is what makes Doom Bolt dangerous.
A very good counter to warlock (the doom bolt guys) garrisons is troll armies. Trolls regenerate; if you bring 9 units to battle and 8 are killed in the process of winning, you get all 9 back at full health at the end. This also works with shadow demons, great lizards, werewolves, etc. Note that warlocks get 4 ranged attacks after they fire off their doom bolt, so expect to take some losses after the first wave of spells. In general, trolls are a great counter to dark elves, so you might search out a city of those first.
A very good counter to warlock (the doom bolt guys) garrisons is troll armies. Trolls regenerate; if you bring 9 units to battle and 8 are killed in the process of winning, you get all 9 back at full health at the end. This also works with shadow demons, great lizards, werewolves, etc. Note that warlocks get 4 ranged attacks after they fire off their doom bolt, so expect to take some losses after the first wave of spells. In general, trolls are a great counter to dark elves, so you might search out a city of those first.