I have finally managed to kill him too
but with a warrior after all, and i have used two kind of fairies, dryads, inq, ang greend dragons.
fairies do insane amount of damage with whip+poison dagger, i have seen 50k+ krits at lvl 23. (not in this battle as Karador have some defence) The whole main battle was not lasted 10 turns. With some time back and resurrect it became a no casuality victory.
The only spells i have used are bless+resurrect+fire damage to weapons.
Warriors seems to me more powerful than a mage really.