I tried that fight for fun and i don't remember it being that hard; it took a very very long time because it was difficult to inflict damage on him but there was never a chance of death. He hit for peanuts and I guess my characters had decent resistance because i don't remember his spells being that bad (i played on normal so ignore if othewise). I think in the end I was able to blind him a few time which gave my warriors auto hits; well 100% hit.
Thats strange...my warrior was 2K hp and full res with high def rating and he kill him with one turn. maybe my game was bugged?
I can printscreen the spam if someone want.