FFX: Yunalesca. Just... just Yunalesca. Also Seymour Flux to a lesser, but equally devastating degree. Took so many tries.

FFIVDS: Does the whole game count? Well, Barbariccia and Golbez. They REALLY got buffed. I even ran out of Phoenix Downs for the Barbariccia one. D:

FFXIII: If anyone remembers my bitching about the final boss...