By the way, I had the same thing happen to me in the beginning: Rutger pops up and wreaks havoc. And they have a couple of other powerful heroes there, I don't remember who. I'm actually at that mission right now. It's funny that Zehir is level 20, but Chaos hasn't been offered to him yet. He has one free slot, and if it's not offered soon, I'll take Darkness. Otherwise, he has Air, Light, and Summoning, and he has Phoenix, Holy Word, and Banish, plus some general Light spells, so it should be fine. The only problem is his combat stats are weak.

Darkness is a useful skill; out of 4 heroes, it's available to 3 of them: Godric, Faidaen, and Zehir. But I didn't use Darkness in the battle between Zehir and Biara; I just didn't have time. I cast Banish twice and Resurrection once, and that was it. But Darkness is useful in all other battles, so it's up to you.
My Zehir also has poor attack and defense, especially attack. After the 4th mission, I barely managed to raise his defense to 15, but his attack is only 11. So, no matter how hard you try, I don't think your stats will be much better. For a mage, everything goes into power and knowledge, and if you pump up his attack instead of defense, his army will be completely destroyed