You don't need Valefors powerful overdrive to get 9999. Trust me, Valefor and Ixion is that powerful. I ahd them doing 9999 by Macalania on my weak save. ^_^
Well done on beating him, but you have to face him one more time during the game. He's a lot easier then in my opinion, though releasing just overdrives on him is not going to win the battle.