To get easy potions, go to where you battle Riku, talk to him, and he'll offer you to battle say yes. The first thing you should do is run forward, lock on (R1), then attack. After hitting him 4-5 times he will "fall" backwards then spring forwards and hit you, but there is a way to dodge it, when he falls backwards instantly move to your right or left, then attack again. He will sometimes say something to insult you, to make you come at him after 2 hits hit him one more time then he'll say "my turn" jump or run to your left or right, then you hit him again.
You will know if you are getting close to beating him, because he will fall backwards with less hits get out of the way again (it takes half of your HP), he might even do the blocking thing again but with less hits, while you are this far in the battle you need to listen to what Riku is saying, or you will lose, he will sometimes (if far in the fight)just block once then strike. Again, listen to him for the warning.

When you beat him you will obtain a potion every time you beat him, so fight him often before continuing your mission.