haha i beat Lady Yunalesca with a few holys and one overdrive from a chosen Aeon. heres the battle in steps

Tidus: Hastega
Yuna: Holy
Yuna: Holy
Lulu: Flare
Yunalesca: Cant remember
Yuna: Summon Shiva
Shiva: Haste
Yunalesca: ???
Shiva: Shell
Shiva: Boost
Shiva: Boost
Yunalesca: ??
Shiva: Diamond Dust

Thats basicly it for each form it was Something like that.