You might want to put spoiler in the title of this
Anyway, I had the exact same problem, so I asked on the usenet newsgroup, and some guy replied with this, which helped:
<< This is a fun battle though it helps when you make some progress. I
was switching between this and Die Hard during the cut scene. THE
frustrating scene was after killing the boss on the airship with no
save game before busting up the wedding.
Finally, my routine for this fight is:
0. Everyone and all Aeons must be in Overdrive .At the last save
point wipe out a few species of fiends. The Aeons will each deliver
9999/9999 HP. Stop reading here and you will easily figure out the
rest.
1. Lulu casts Bio on Seymour Boss. Maybe not necessary
2. someone casts Haste on Yuna.
3. Yuna goes nuts with Summons.
This does require a little luck. Someone must survive Seymour's
Cleave/Zombie/Full Life ( Yuna and Lulu have high evade numbers but
for me they hardly ever survive cleave ). Then that person uses a
Mega Phoenix on the KO'd members. Using the above steps Seymour never
gets to call Total Annihilation. Keep an eye on the round sequence
making sure Yuna gets in a summons call before Seymour gets in a combo
sequence. If you get more than one Yuna round in a row call Shell and
Protect on other members. I am not sure this is necessary.
>>
The key is the summons. Before the fight, it is very important to make sure every Aeon is in overdrive (or all but one if Yuna is in overdrive). Plus, he didn't mention this above, but using Hastega on the whole party is a smart move. If you don't have that, just haste Yuna and have her keep with the Aeons. Bahamut alone should do over 14000 damage to both Seymour and that thing that helps him. Also, if you have the Mighty Guard skill with Kimahri, try to use that. Have him in the party to start, and cast that quickly.