Impossible, Mage - No unit loss.

[Quelthos]

At least up through level 20 or so damage spells are very much viable, but only if you use them in the right situations. If an army is bundled up in five stacks of large troops, geyser becomes a lot less effective than manipulative spells (for me these are slow, trap). But if an an army is in 8+ smaller stacks, it's great.

I have only faced a few stacks of black dragons to this point, none higher than 10, but my strategy against them has been to space out all but two of my units, so that the dragon will be enticed for double damage, and lay a trap in front of those units. Then during the turn I rearrange my troops again and lay down a second trap spell. Meanwhile my range/no retaliation non magic users lay into them.

Just as it is important to reshuffle your army in impossible modes, it's important to change up your spell tactics.