Ok, here's my personal feelings on the best way to fight Demyx:
- Make sure ahead of time that you have the Explosion, Aerial Finish, and Slide Dash abilities equipped.
- Every time Demyx calls out his clones, use Valor form right away. I wouldn't suggest wasting drive on Demyx himself.
- Just like everybody else has said, make good use of the reaction command "Wild Dance".
- After Demyx uses his water dash across the stage, go over to him and stand about 5 feet away from him. Demyx will then bring up a circle of geysers around himself. The second the geysers start to fade, press X to slide dash him.(instead of standing 5 feet away, standing right up close to him actually works better. unfortunately it's not always possible to do so, considering the limited time you have after he dashes. if you're not close enough, then dont try it.)
- Do not use Wisdom or Master form in this fight. I know Master form is very tempting, considering that at this point in the game, you didn't get it all too long ago. Just please refrain from using it, it's all for the best.
- When Demyx yells "C'mon! Kick to the beat!", he will start dancing and and bringing up lines of geysers around him. When this happens, the best strategy is to use both air combos and Aerial Recovery to attack him from above. Try your best to stay directly above him during this strategy. Avoid moving too far to his sides while attacking.
- Unfortunately, I have not found an effective way to control Demyx's reaction command "Steal the Show". Basically, this reaction can turn out 2 ways: 1. You press triangle when the reaction comes up, and Sora dances behind Demyx for a while before smacking him with the Keyblade. 2. You press X when the reaction comes up, and Sora taps Demyx on the shoulder. Demyx then smacks you with his Sitar, and he takes no damage. Obviously, the problem with this is that the Demyx fight is a very fast paced boss fight, so you're likely caught up in the habit of smashing the X button.
- When Demyx just yells "Water!", just get as far away from him as possible. He's going to start raining down huge water balls on the stage. You may get wet.
- When Demyx starts shooting water balls straight at you, move to your clearer side and run forward, attempting to slide dash him. Do not jump up and attempt to hit him with an air combo. If you do that, you will get hit by the water balls.
NOTE: I am not implying that this is the only way to defeat Demyx. This is just the way that I've found works best. If you have a different way of fighting him that you think will work better, then go ahead and try it.