okok well best way to fight seymore flux is use every singl aeons over drive, and use mighty guard over drive from kimahri....i did it and i only had to use two characters over drives, the aeons take about 60,000 life using all ther eover drive's, maybe more i don't remember, but they should do 9999 each and bahamut should do more
you can get the aeon yojimbo(that is his name) by paying him 250,000 just keep offering that until he accecpts, it will work...that is the only way, you have to pay him a large amount of money no matter what, 250 is the lowest i know of, you can try lower if you want to, but i doubt he will go below 200,000