My paladins struggled with this guy, in particular the part where you run to the new hole he emerges from. My problem initially was that if he was casting the poison spit at the leader, it would hit the followers as they were running behind me. I switched to just using IWT to make everyone run to the new spot and that fixed it for me. Still tricky not falling down into the hole with him, but stacking everyone up and just strafing the whole team when spit lands was enough to get me over the hump on normal. I haven't been back on any harder setting, so I'll defer to others on strats for that.