You will take your vengeance upon his monstrous form. He is almost invulnerable unless you do the following. When the glowing yellow bands surround you, find the gong that is emanating them. You have to hit it either with your Reaver or with a Tk bolt. This will stun him. Get to him and hack at him as fast as possible. Repeat this as needed. Watch out for his other attacks. Near the end, all four gongs will start to emanate. The two end ones should be hit, killing him and giving you stronger telekinesis.