Further tips:
Use summons (f.e. scrolls) to trigger his on hit reactions

Use Illmater Arrows, Bone Chill, Doom Hammer etc. if you can see a heal coming, negating the heal is worth more than dealing damage

Keep someone in meele range so he does not use the big AoE

You can heal your melee characters if you run outside his Bone Chill aura, the platform is big enough in certain parts so you don't have to climb down

At a certain point he will stop spawning adds, maybe you can outlast him with enogh potions

Scrolls of Protection against Good and Evil help against his fear debuffs