Like Alastor12 said, when he launches a fireball, you hit it with a sword and deflect it back at him 'til he dies. (It takes 3 hits on Normal +, 1 on Easy). And if you're facing the one in the Hidden Bangle special mission, just run up to him when he's not breathing fire and slash his skull to bits until he dies. Then you'll have to fight 2 Shadows/Frost.