your party should have one member that have a capture weapon i prefer Wakka
and u should have Rikku and Lulu << u can not change them
now every time a 3 puroboros will appear and u have to capture just 2 for the arena monster dude
first let Rikku use (shadow gem) on them, that will make them all lose half their HP and they will not grow bigger from the attack cause if u attack them in a different way they will grow bigger up to 3 times and then explode in your face causing death or damage to u and u don’t want that to happen
now every puroboro have half HP, let Lulu attack with Blizzara cause u don’t want o finish them off just yet, the monster is so weak let wakka attack normally and here u have it! the monster is captured 🙂
hope that helped, if u have a different method please tell us .