I think I noticed something *spoilers*

[Cruise Control]

2nd and 3rd Seymour fights are easy.
1) Charge aeons into Overdrive (before the battle, use boost)
2) Charge yuna into overdrive (optional)
3) Poison seymour somehow
4) Summon aeon, and use overdrive
5) Repeat with all aeons
6) Watch his HP go to 0 before you can blink
Result:
1) Seymour takes damage
2) Mortibody dies, steals some of seymour's HP to be reborn
3) Seymour Dismisses aeon, and takes more damage due to poison, and dismiss uses up his turn as well
4) This makes you a seymour killing machine
5) Jump up and loudly yell "Up yours stupid b****" (referring to seymour)

[fire_of_avalon]

I just beat the game for the first time a few days ago, and I assumed it was Seymour. I knew that jerk wasn't gone.