Diabo Va Thall roots himself. He periodically shoots two flaming arrows at you that do 25k damage each. The real threat are the Shissar invaders. The Shissar move very slowly, and the safest way to fight Diabo Va Thall is to lead the Shissar to one side of the room, then run around them and beat on Diabo until they catch up, then run to the other side of the room and repeat. Classes with strong ranged damage may find it easier to circle Diabo (staying out of range of the Shissar, of course) while zapping him with spells or arrows.