YAY! Someone can use MY help for once! I am so happy! *sniifle*
Ok. so Demyx is: Annoying to fight, annoying to listen to, and this fight will leave you screaming in agony whenever anyone says: DANCE, WATER, DANCE!!! Slyguy is more affected than I.
He will start with 50 water forms to beat in 40 seconds. Smack one, and then use the wild dance to take care of the others to make it in time. His most frequent attack is to charge accross the field, leaving huge water pillars behind. He always follows this up with "Like it?" and surrounds himself with water columns. The best way to handle this, is stay far away from him, and in his short pause before saying "Like it?" charge up to him, and start smacking him around, along with occasional shouts of Shut the "f" up! His other attacks are shooting you with bubbles, which can be avoided, "Water" signifies a rain of bubbles (first a pink scythe, then flowers, then bubbles?!?!? these guys are FREAKS!) "C'mon, kick it to the beat!" signifies water towers, along with an unparryable physical attack combo, so dodge as needed. Than there is the dreaded DANCE, WATER, DANCE!!! This is the toughest attack of his. you must defeat 10 water forms in 10 seconds, or...well you lose. I have no idea WHY there is a time limit...
After he starts, just battle him, and stop his attacks as needed, getting in damage whenever possible. drives are too dangterous to use, with little chance of doing damage, and the possability of turning into anti-form... Also, if he uses the water forms while you're in drive, you're screwed. Donald and Goofy are essential to dealing with them. use magic as needed, mostly for cure or reflect. I haven't really experimented with moves, and ways of winning, but he is easy as long as he either doesn't use the water forms, or you can deal with them easily. potions are good, and elixers are even better. If any reaction commands appear, use them. The show stealer? allows you to temporarily stun Demyx and get in more damage. Eventually you will win, I personally only lost because of the water forms...GAH! Oh yes, try to preserve your HP as much as possible, you might need magic to save your butt! Trinity limit or goofy's limit are always good ways to deal maximum damage too.