There is no way for the characters to hit for more than 9999 with a single strike. There are only 5 attacks that only hit once that can do more than 9999 damage: Shockwave Pulsar, Kamikaze, Eden, Chocobucle, and Cactuar at level 100. That's all you get, you can't attack for more than 9999 unless you're playing FFX/X-2.