You are suppose to kill him. What level / class are you? If you are a warrior you can drink a shadow potion and use your spin attack near, but not directly on, him. Survivor; use scorpions, or sneak / ranged magic attack to remain hidden. Mage; maybe Curse, then Freeze, Blind, Hell Spikes..... Poison Weapon/Skin/Cloud/Arrow and a strategic retreat may work, possibly using one of the curse skills if his poison resistance is high.... A bow my help as well, if you are having problems melee....
