Blitzball is the one of the biggest sidequests in the game, with the main goal of it being to earn Wakka’s Overdrives and the Jupiter Sigil. You will gain the ability to freely play Blitzball after finishing up the Luca portion of the main story, where you can access it from any Save Sphere. Normally, only the League option is available, but if you exit and re-enter the Blitzball menu multiple times, the Tournament option will become available. Win the Tournament to gain this trophy/achievement.