Wait for him to materialize and hit him. You need soul reaver to do damage. The spirit forge in the center of the arena always automatically fills your health. Simply run over to it to accomplish this. When you strike him the first time, he appears on the second tier, again usually behind you. Finally, when you hit him on the second tier, he will materialize on the third and highest tier, again usually behind you. Quickly hit him a third time to defeat him.