alright ppl, thanks for all the help!
i have been grinding a little, wich i somehow managed ^^.
with alot of Haste, Slow, Power Breaks, and 'Speed break' i manage to outlive the machina on the bridge!
but i have still not encountered a team with a flamthrower, i think they are simply not there anymore.

after that i tried another turn to defeat Seymour but that failed.
i know i should have Rikku on my team becaus of her Al-Bhed potions, but beside those she has nothing to offer so ive replaced her with Tidus.

i think in the end its all about being perecise in yr movements, and being very lucky :o

since i am not a person that will grind an hour, i will grind and try to beat seymour everyday.
sooner or later ill outpower him, or be lucky enough :P