This really isn't too hard and you're bound to get it as you progress in the story. The easiest way is going to be to use an aeon's overdrive on an enemy that has that element as a weakness, but as you progress in the story it will become more likely you'll get 9999 in one hit because you'll be hitting the damage caps.
I personally got it by using Ixion's overdrive on Crawler once the Negator was defeated via either Wakka's normal attacks or a Ronso Rage of Kimahri's (Seed Cannon worked well to one shot it).