I've got the full Vegas over here on my end, and let me tell you...it is MUCH better than Movie Maker. Probably do to the fact that my computer isn't built for making videos, but it still kicks butt.
Riku on Floor 13 was the absolute worst. If I ran out of 0 cards when he pulled out his Dark Aura slight, I'd just Dodge Roll like a maniac and pray he didn't hit me.
Larxene also was hard to deal with. Especially with that Lightning Bolt sleight. Ugh...
Darkside was a real bugger, too. Took me all bloody day just even get a decent bite in his HP.
What I would suggest for you is for the first bit of Water Form bashing is stay close to Demyx as you wipe out all his forms. The Wild Dance reaction is a M-U-S-T here, in my experience using anything else takes too much time. Plus, this helps you to get some early strikes in.
After that, pound him with Trinity Limit or Goofy's Teamwork Limit, then go Valor or Master to recharge your MP and keep him airborne as much as you dang well can. When you're all out of Drive, then just lather, rinse, repeat. Beware of Anti-Form though. At this point in the game, chances are high you'll go Anti here, and while it's got some nasty combos, you take extra damage if you get hit. So...be advised.
On the basis of dodging his attacks in case you can't go Valor or Master yet and you're waiting for your MP to come back, listen to his taunts. "Dance, water, dance!" usually means more Water Form bashing. "Come on, kick to the beat!" means he gets surrounded by geysers. Just keep close and jump around him to dodge it. "Water!" usually means a bunch of high speed bubbles. To avoid, just run backwards.
