Ya know, I got stuck here too. XD This is where I realized that I need healing stuff actually.I'm pretty sure you have cure at least. If not Cura or whatever. But, if you do, here's my stragety. Start by equiping Ethers and potions. More ethers though. Maybe elixers if you have them. Okay, so when the battle starts, don't even looka t how much time you have. just keep using major combos, and just mash the triangle button at the same time. This way, you can attack efficiently. Now, when you are done with the clones, focus on airial atacks. They are most effective against Demyx. Use combos, then run. But listen closely to what he is saying. If he begins to play his sitar, run real close. He's about to pull oput more clones. This part may be rough, because you have like... 10 seconds. again, mash and combo. Your drive is the last resort, use it only if you have no ethers, no potions (I said potions with healing magic, because while the MP bar is carging, you can still heal.) and your MP bar is just starting to charge. Maybe have Auto-valor equiped, just incase. Wisdom form will also help. Just gain some distance, and shot him. But remember, only use drive if you are completly screwed. Think before you react. Okay, to continue, when he has one bar of health left, heal. No matter how much health you ave, unless you have full health, heal. Maybe go into form here, and pray to god that you don't get anti-form. He will most likely summon more water clones, in attempt to make you lose and **** you off because he has such little health. That's why valor MAY come in some use here. It's fast, therefore, you can dispose of the clones quickly. After the clones are done, just use everything you have on Demyx. He'll be down in no time. Also, make good use of your limits from time to time.
Okay, if you are talking about the underworld battle, you are an oblivious nincompoop. If not, ignore this sentence.
And, again, only use drive if you have to.
ONLY USE DRIVE IF YOU HAVE TO!