Yup, he is one of the top reasons why my good chars die. But you can best him if you got a little luck and:
1. Dip healing liquids and troll bloods on cans. Like this they don't break at enner's scream.
2. Get a loooot of wands. Striking, lighting, fireballs, even acid rain. They are the key to this fight.
3. When you get to his level, immediately use Scroll of Detect Material, and detect for enner beast flesh. If you don't have one, you probably get one cheap from Attnam library.
4. IMPORTANT Never, ever and ever get into melee range of enner. His scream will do amazing amount of damage then. Just stay away from him and zap your wands empty. Don't worry if you get panicked. Just heal yourself full, you can still use wands.
Enner is actually fairly weak, and you'll probably get him down by couple of strikings. Good luck for your next try!
Right, now you killed me. Happy?