FFX-2: Overkill Achievement
Deal 99999 damage with one attack
Similar to the corresponding achievement in FFX, you must have the Break Limit ability to deal over 9999 damage and get this achievement for hitting the new max of 99999. There are a number of ways to get this ability. Check the FFX-2 section regarding Break Damage Limit here to get one.
Once you have the ability, you'll want a strong attack that ignores defense (the Mascot dressphere is good for this) and then buff the appropriate stat through accessories and in-battle buffs, and unleash a powerful attack on a weak enemy, such as those found on the bottom floors of the Via Infinito.