I used to suck too. But now I can't stop winning. I think I got a 10- 0 win once. All I know is that I have:
Tidus
Wakka
Brother
Kyou
Zalitz
uh... someone...
Miyu

And Brother is at level 24 or something while everyone else is at level 12 or so. I even reset the stats to make it harder ( stat fom beginning) and I can't lose!
Yet I keep losing the Tournaments. How do you win them?