Wait, where it's still Riku-looking?
MAKE SURE you have the Olympia keyblade equipped, then, equipm Guard, Counterattack, and Ars Arcanum, when he attacks, guard, then counterattack. When he uses some kind of Strike Raid attack, if you were good at fighting Wakka, guard it to send it back at him. When he does the zoomy flying attack, jump as high as you can and glide around. Also, use Ars Arcanum whenever possible, it deals a LOT of damage and easily whens you the fight.
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Fenix kicks *** for the sig and avy.