How do you beat demyx off of kingdom hearts

I hated this guy.
The trick is to use the reaction commands and fire spell. each of these kill a few on every use.
then atack him to bits and repeat

edit:bellow is right. I've just never used limits before. Always thought they were lame

--
Edit: I'll leave the other person's answer up top but I've found a much simpler way to beat Demyx, even on Proud Mode. Goofy and Sora's joint Limit, Knockmash is EXTREMELY useful in this fight, as Goofy locks on to Demyx and depletes almost 2 HP bars!! It's truly amazing and very helpful. Also, if you're weary on getting up close and personal to fight, you can always use your Wisdom form and shoot him far away. Valor and Master work well otherwise at getting lots of combos in. If the Time Limit in which you must kill his water clones is stopping you from winning, use either reaction commands to defeat the forms or Donald and Sora's Limit. Either way, go into this fight prepared. Make sure Donald and Goofy have the latest weapons and make sure most of your Abilities are equipped.