What equipment do you have? Equip a weapon that does cold damage or magic damage and just pound him to death. Anything that does fire damage is pretty much useless. Take a bulls strength potion and a cats grace potion to increase your damage and your ac, use whatever monk powers you have at your disposal. Also, are you using a henchman? If so, give them the same potions.
Don't worry about his magic attacks cos chances are that eventually he will turn himself into a chicken anyway lol (he does that a lot when I fight him