How far are you on the grids? Usually I have Tidus almost to the end of his (Quick Hit signifies this), and Yuna already knows Holy.

My strategy is to have Tidus cast Hastega on Party, and Yuna cast reflect. Use Holy a couple times, and bring in Bahamut (The stronger Yuna is, the stronger Bahamut is, or any aeon for this matter). If all your Aeons are in Overdrive, they should do near or at 9999 damage, Bahamut hits for more in his OD, and Valefor can also if you do the capture sidequest, then power up her Nirvana before hitting Gagazet. Valefor will hit for more then 10K with Energy Blast.