Dance, water, dan--*shot*
Demyx is pretty hard to beat--but if you think beating 50 in 40 seconds is hard...further into the fight, he'll pause the battle and summon up ten water clones. You have to beat those in ten seconds or you lose the whole fight. Very frustrating, to say the least--especially the first time when you aren't expecting it. To beat the clones, I normally just hit the one closest to me, which enables the reaction command. Then you use it and when you drop the clone, hit another one and repeat the process. I think I usually rotate the control stick when I'm doing this so that you more or less just spin in a circle so you don't go too far away from the rest of the clones (though they'll swarm you anyway o_O). Then when fighting him, try to corner him, but when you go in for attacks, make sure you're in the air as most people get more combos in that way and you'll do more damage with each string of attacks. Plus, you're faster in the air and otherwise he'll probably bubble you to death before you get close enough to hit combos. XD Then for the 10 in 10 seconds...well, that's why it's best to corner him because that way they're all right in the corner so when you pick one up, you can usually take out all or most of them without too much trouble whereas otherwise, they might scatter too much to be able to do it as efficiently. :x
Rofl, it took me 15 tries to beat him on my Proud Mode file since most of the time I didn't want to hit him. He makes that pathetic little noise every time you hit him and it makes me sad, so I...er...don't hit him and end up dying fairly quickly. XD;;
