While in Destiny Island, train Sora to level 10. Do this by fighting Tidus, Wakka, and Selphie. Plus beat all three and face them all at once. If you beat them you get a potion each time you win. Once you get to level 10, face Riku, if you try facing him under level ten, you'll lose life quikly and make the score 1-0. If you beat Riku get a potion.