assuming all your aeons are in overdrive.....
Cast hastega, and for the love of god put the protect spell on as many folks as possible. sure he'll just dispel it eventually, but i'd rather take 1000 from a crosscleave as opposed to 2500. When he starts charging for the total annihilation attack, keep wacking the chariot because this can delay the TA attack somewhat.
also keep in mind, seymour is not immune to silence. just wittle him down to 25/30k hp, and unleash Bahamut's mega flare.