The most powerful and capable summoner in Spira, and one of the most powerful anywhere, versus an incarnate shell of Malice walking the land.
Freaky Friday.
Yeah. It was decided by a bored Lavos, Ghaleon and Yunalesca that this would be the best way to have this match decided. So Yuna spent most of the time running away from an infatuated Killer and wishing she could speak effectively in this new body to explain the situation.
Whereas Lady just summoned all Yuna's aeons, went into Mascot Yuna mode, and proceeded to level half the city while singing along to "Barbie Girl".
The judges, in fairness, were never told to make the match fair or even non-destructive.
They decided to level the other half of the city.