Seymour

Seymour? *Yawn*
Tidus: Haste Yuna
Yuna: Summon Valefor
Valefor: Overdrive
Valefor: Dismiss
Yuna: Summon Ifrit
Ifrit: Overdrive
Ifrit: Dismiss
Yuna: Summon Ixion
Ixion: Overdrive
Ixion: Dismiss
Yuna: Summon Shiva
Shiva: Overdrive
Shiva: Dismiss
Yuna: Summon Bahamut
Bahamut: Overdrive
Bahamut: Dismiss

It pretty much goes like that every time, with varying numbers of Aeons...end result, Seymore defeated once more without doing much more than scratching you. Yunalesca on the other hand...