As long as you keep close to him and stay at the top, all you need to do is homing attack him until he's done. I beat the Nightmare Reize challenge 1ST TRY for goodness sake.

@RyanThe1st: Yeah, Dash Slashes make the boss a cakewalk. (And I only got hit 3 times fighting him the first time, I think.) Haven't tried the challenge yet, though.

@JonRetro It's the same thing, but you have three health and 80 seconds.