First, it will make the Sin battles (and the final bosses) much more easier if you have the following abilities: Curaga, Hastega, Doublecast, Ultima, Flare, Quick Hit, Full Life, Auto-Life, Copycat, and Entrust. The Celestial Weapons also make it easier.
Tidus=Caladbolg
Yuna=Nirvana
Auron=Masamune
Wakka=World Champion
Kimahri=Spirit Lance
Lulu=Onion Knight
Rikku=Godhand
Some Weapon/Command abilities also help. For example, Equip Yuna and Lulu with weapons bearing One MP Cost and Magic Booster, and teach them Doublecast, Ultima, and Flare. Also, have Rikku in your party with a weapon that has the Gillionaire ability. From Sin on, you'll find that with a weapon like Godhand, she'll make 100000+ after each battle.
Start with Tidus, Lulu, and Yuna.
Cast Hastega, then have Lulu doublecast Flare or Ultima, and have Yuna do the same or make her cast Holy. (It helps if Lulu has the Onion Knight and Yuna have Nirvana) Then if possible have Rikku Copycat it. (<-- Works best if Rikku's magic attribute is high) Have Auron, Tidus, and Wakka, and Kimahri use their overdrives. (Masamune, Caladbolg, and World Champion are all useful) If Kimahri learned the Nova Lancet ability, use it when he hits overdrive. If you can, use Rikku's Spare Change ability and toss 100000 Gil for 9999 damage. Have Rikku Mix for Trio of 9999 or Supernova. If Sin's overdrive guage gets to 3/4 full and over, have Yuna summon aeons that have the Break Damage Limit (Anima, Bahamut, and The Magus Sisters. And if you have the celestial weapons, (Nirvana, World Champion, Spirit Lance, Onion Knight, and Masamune) use Valefor, Ifrit, Ixion, Shiva, and Yojimbo)
This strategy hasn't failed me yet. You should try it. :D :p :) :cool:
