Reading from this thingy:
Like Captain Hook, this opponent is deft of blade. Easier than the dragon? A little, but not by much.
Deflect his attacks with the Guard ability. The Counterattack ability enables you to recover quickly and deliver a whole series of attacks before he can counter or evade. The Olympia keychain will significantly aid in this strategy. When you need to cast Cura, always glide away to make some space.
If you knock Riku off balance, select the Arcanum command (which appears at close range) and let him have it!
EDIT: You get the olympia keychain from winning the phil, pegasus, and hercules cup tournaments.