This should help you with the rest of your boss battles:

Aeons = Overdrives
Wakka = Overdive
Tidus = Overdrive (hopefully you know Blitz Ace soo or by now, but Slice & Dice will work also)

Go in battle, do the switch with any players you want exp for, have them guard or whatever, then bring in Tidus, Wakka, and Yuna.

Have Wakka do attack reels, Tidus either Blitz Ace or Slice & Dice, then have Yuna summon all Aeons to do their OD, Preferrably saving Bahamut and which others that can deal more then 9999 damage til the boss is down to 10K or laround there.

Poof any boss should die, exception will be, well you will know when you ger there.