Knights of the Round which can only be acquired by breeding a Gold Chocobo cause its located on a island only accessible to the GC. You need good prep work though, Ultimate Weapons help and High armor and Level 4 Limit Breaks. You may also need W-Summon (acquired from the Battle Coliseum same with Omnislash Limit Break) and the Mime Materia (acquired with a Blue or Black Chocobo). You may also need the Final Attack materia and either Phoenix or a mastered Life Materia. You will also need the Underwater Materia (Use Morph on a Ghost Ship Enemy located in the Junon Reactor, in the area that is underwater. You will get the Guide Book which can be traded to the Kalm Old Man for the Materia and it removes the Time Limit from the battle.

The basic strategy is to Double summon KoTR and then have your party Mime the attack. Final Attack Life/Phoenix will revive character who get killed. Ultimate Equipment and Final Limit Breaks will do decent damage (though not as much as KotR). You will probably want to attach a MP/HP absorb on KotR and Mime so you can maintain health and MP during the fight. You could also try the Lucky 7's trick (If HP becomes 7777hp your character becomes Berserk and does 7777 hp of damage with their attacks) or you can try the infamous Cait Sith Joker's Doom trick (Cait Sith's second Limit Break has an instant death attack that occurs randomly but actually works on Emerald and Ruby WEAPON from what I've heard) though that's going to involve a lot of soft resets.