Demyx himself is easy, it's the Water Clones that will end the fight for you. Level 35 should be suitable for this fight. Here's what I'd do. Have The Olympic Crest Keyblade for an added Combo Boost, have alot of Potions. When the battle starts, use Donald's Comet Combo Attack to take out a good portion of the clones, and the reaction command can take care of the rest. Once that's done, Demyx flies around in these water spouts, or whatever. If you leveled up Wisdom Form, Quick Run is useful here. He'll summon some Water Clones, but if you get close enough, you can use the Reaction Command to temporarily hit him. He'll dance towards you to set up a reaction command. If done right, he'll smacked in the air giving you the chance to wail on him. If done wrong, it may kill you if your health is low. It wouldn't be smart to get right in his face and fight him since he summons Water Orbs to strike you. Personally, I like using Master Form during this fight when his health is low to finish him off. When he says, "Come on, kick to the beat", he'll strike with his Sitar. Then he'll say, "Rock it out." If you use Guard successfully, you won't be hit. Then comes that annoying 10 Clones in 10 Seconds bit. This will be the fight shifter. Using th reaction command only is a big risk, that may, or may not work. I use the Trinity Limit Combo Attack for this. You don't have to, but I use it. It gets rid of all them. Hope this helps.