After what I can see, he has a soak +5/25, so first you need to have +5 to hit him (or 25 damage will be absorbed), secondly, he has resistance of 20 to each damage type (piercing, slashing, blunt). So even with a +5 or greater, you'd still get 20 damage absorbed.
How my own sorcerer beat him (the only time I fought him with a sorcerer), was by using the high level summon "Black Blade of Disaster". I was lucky there and killed him fast (2 attack rounds) due to him rolling a fortitue save of 1+40 = fail
However, I think you would have to reply on henchmens somewhat to hold his interest, possible summons (Moderkins Sword, or other), or use some sort of transformation spells - Tensers Transformation, Shapechange etc.
He is very hard to beat, but not impossible, but he is harder for a spell caster then a warrior type in my oppinion. (My fighter/weaponmaster/barbarian just wiped the floor with him together with Deekin, even with Aribeth turning against me (so I wacked her also)).
So basically - use high level summons, henchmen, transformation spells and see if you can meele him down. I don't think many spells does a lot of damage to him, because of his very high save throws. (and because the guy has about 600-700 hitpoints)
Also a little pointer, when you damage him, he turns the whole area into lava/fire - you can then use the grabbeling hand to teleport up to one of the higher platforms, and avoid it.
Good luck, and I hope some of this information helps a little.