His "shadow" is what heals him. Move your character in the arena farthest away from the shadow and Mordac will eventually make his way to you. Then, fight him by running around keeping tight circles while periodically hitting him. When you defeat him, he will disappear and you will fight him in the final battle again. This is the same way to kill his "partner" (the one with the shield) towards the end of the game. Note: It may help to use a "quick" weapon (for example, long sword or mace; if you are using the Monk, you may want to avoid using the "sweep kick" since it takes just a little bit of time to recover from the movement).