How do you defeat Demyx in Kingdom Hearts 2

The battle takes place right before the battle of 1000 heartless. He uses water, so im pretty sure thunder would be of good use to you.
Demyx will start by doing his special move "Dance Water, Dance!" which will make him invincable (You wont be able to hit him at all). You then have a time limit to deafeat his water minions, or its game over. Use the reaction command "Wild Dance" to make this a lot easier (don't try to use wild dance on Demyx, he will not get hurt.).
He will then start going around, using water attacks (you will know because he shoots water out of the ground). He will first glide at you, but this can be easily dodged. After that, don't go near him, he will shoot water around him, and you WILL get hurt. If you are a distance away, he will then shoot waterballs at you, this can be easily deflected with reflect. Also, watch out for the reaction command "Show Stealer" This move will show Demyx playing his sitar, and you hit him and he will be dazed for a second.
Ocassionally, he will use "Dance Water, Dance!" where you have to defeat 10 minions in 10 seconds.
Keep repeating all these steps until he is defeated. In my opinion, Demyx is the easiest boss of Organization 13. You can even ask Demyx, he will agree. Like in the Underworld at Olypmus Colliseum, you will catch him sneaking around. He will see that you are "Roxas" and Sora will ask what he is talking about. Demyx completely ignores Sora and reads a notecard then says," If the object fails to resond, use aggresion to illiberate his true disposition. I told them they were sending the wrong guy for this!"
Have fun and good luck defeating Demyx!