Hi!
I also had trouble when I fought Seymour Flux. It took me at least 10 times to finish him off (I'm really hoping I won't see him again!!).
My GFs were not that powerful at all, so I think there is no way I could have beaten him with my GFs.
My characters are pretty weak too, only Auron has
more than 4,000 hps.
The way I beat him is:
(1)Use Lulu's Bio on him as soon as you can, and
keep him Bioed, this way, you won't really
have to do much fighting, Seymour will just
die after a while.
(2)Every time it does a dispel on you, it means
his next move will be the cross something
combo, what I did was, every time AFTER the
dispel, I will cast reflex on my strongest
character, so he cannot be killed by full-life
after he got zombied.
(3)Use Auron's armour break and power break on the
machine, and cast haste to everyone, and keep
taking out the machine. This way, if Seymour is
Bioed, it would cost him 2000-3000 HP each time
he revivies the machine.
(4)Just use yuna as the healer to keep everyone's
HP as high as possible.
Good Luck
Didu