I would say in my opinion, for him to use knockersmash as many times as he can. It does a high amount of damage so use a few ethers to keep mp recovery short. For the clones, just button bash triangle and take down the last one quickly.
It may not be the best strategy, and I confess I've never tried it, but I think it should make the fight easier. Since hitting him gets harder and harder as the battle goes, i would say to use knockersmash after the fight's gone on for a bit, then use them after the second wave of clones are gone.
If the 2nd wave of clones still pose a problem, then (although i've never tried this as well) when he thinks the clones are about to come, he should use the final attack of fantasia straight away. Fantasia's final attack rapidly hits and covers a wide circular area so it might destroy the majority of the clones.
If he still can't beat Demyx then you should probably ask for someone else's opinion.
Sigh, every forum I use turns into a war zone. I kinda miss this place. I might pop in every now and then, but there are so many scars already, I don't feel like I have a home here.
Keyblade Master: AKA Shinaka
April 2005- Feb 2007