If you want to straight up fight him, use your strongest earth spell, however this is no easy task. What I did was poison him and safespotted him. When the poison starts to run out, repoison him. I'm just having problems getting to a safespot without him disappearing, do you remember what safespot you used ? I don't remember how exactly, but I got him trapped behind a couple monsters. A giant rat and one of his damn skeletons he summons.