First fight Wakka until you get to about level 5 or 6. He is the easiest to level up on because if you hit the ball back at him when he throws it the you get 1 EXP per time. Then battle each of them to get the 3 on 1 option then battle them all. Try to get Wakka last so you can get some extra EXP. This will get you some potions. If you still want to level up more then I would either battle Wakka again, 3 on 1, or Riku because he gives decent EXP and potions.
Added by: Venxas24 Oct 22nd 2012, ID#34146