Impossible, Mage - No unit loss.

[Quelthos]

I was able to defeat Karador at level 24 with only a stack of 5 black dragons.

Karador Setup

Karador Midway (No Crystal, No Mana)

Karador Victory

You can see I have a lot of damage modifiers, from my helm to the two firestorm artifacts to level 3 Destroyer. So I opened the fight with two max level armageddons, which reduced my stack of dragons to 1 and wiped out all necromancers, ghosts, skeleton archers, and all but 2 bone dragons. I used Time Back the next round to raise my stack of dragons back to maximum. From that point on it was a combination of a level 2 fireball (since my fire damage was so amplified), a level 3 healing spell (can't beat 1 mana), the dragon's special ability and just flying around the map to avoid getting hit.

On the left and right sides of the field are squares where only one creature can reach you, so I would take turns there with Glot's Armor as another means of avoiding damage. I used Charges to get some extra mana back as well. I used a second Time Back later in the fight as a means to kill the crystal.