Well, since the quests do not count in this campaign, you may as well skip that. I have never tried doing that "task" (in champion of the arena, there are tasks, not quests) because in my opinion it looks impossible to beat the game using only those units provided by the school of gloom, especially on higher difficulty.
Maybe one can try to play with a tough tank unit and use the stone skin + magic spring + healing + teleport in k'tahu's weak spot tactic. However, once some gorguanas appear, you're bound to take some losses, and if they are not quickly disposed of, the game will surely be lost.
Another tactic might be - put your tough tank unit in a corner, stone skin, magic spring, healing and flame arrow at K'tahu.
Problem is, there are few to no decent physical resistance units in the main area.
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