FINAL FANTASY X/X-2 HD Remaster

I've found the easiest way to unlock this prior to the endgame grind is through an Aeon's overdrive ability. Aeon's damage is based on Yuna's magic and strength stats, so be sure to keep her active in your party throughout the game to level her up. They will hit 99,999 damage with an overdrive long before your regular characters do.

In order to unlock the break damage limit on the aeons and do 99,999 damage, you need to power up a certain character's ultimate weapon for the first time. For the achievement, only 1 hit of 1 aeon's overdrive would need to do the 99,999 damage, so not all of these are needed.

Valefor - Yuna's Nirvana needs to be powered up with the Moon Crest

Ifrit - Wakka's World Champion with the Mercury Crest

Ixion - Kimahri's Spirit Lance with Jupiter Crest

Shiva - Lulu's Onion Knight with Mars Crest

Bahamut, Anima, & Magus Sisters start with the break damage ability.

If you have any additional questions, please ask in the comments and I will quickly get back to you. If you negative vote, please say why so I can address the issue, and if I helped you out, please give a like!