Every Aeon full overdrive.

Have most of your party really incredibly good. Make Yuna, Lulu, and Tidus awesome. Might help if they all had overdrives. Make her aeons strong.

You will know when you are prepared for the battle. Make sure sphere grids are awesome. (full)

Strong Aeons, all overdrives going to be very useful. DOesn't matter when you got them during the game what matters is that you have all of them strong, maybe a bother but it will be helpful in your quest to finish the game.