~Defeat Nemesis~

Nemesis is the most powerful boss selectable in the Monster Arena and was the most powerful boss before the International Version of the game. If you're not using Yojimbo it's a decently long fight. Your party should have maximum stats, yet he doesn't need a high luck stat to hit. To fight him you'll have to capture 10 of every fiend in all of Spira; and then defeat all of the Species Conquest, Area Conquest, and Original Creation fiends in the Monster Arena. Once you've done that talk to the Attendant and he'll unlock Nemesis for a fight in the Original Creation list. Whether you choose to use Yojimbo or not, once it's dead you'll earn this trophy.