Fight
with all you've got

Dark Break - Jump up and slash downwards 5 times.
Dark Firaga - Homing fireball.
Dark Aura - Teleport around & attack, followed by explosions. Yes folks, this is the same attack he used on Sora in Kingdom Hearts.
Last but certainly not the least, Riku can call upon the help of his good friend the King to perform:
MM Miracle - Restores HP & stuns/damages all enemies