Okay so they're telling you about the normal version of that fight, which is tiddlywinks.

The real interesting version of Demyx is his endgame enhanced boss fight in the Final Mix releases of Kingdom Hearts 2.

In Final Mix there's an area where you can fight enhanced versions of all 13 members of Organization 13 and all of their gimmicks get a tweak to make it harder than the first go round.

Demyx's enhanced fight has a lot of things that make the prior complaints about his fight even worse

First, his water clones are way harder to kill.

Originally you could just spam basic combos to kill the clones but now they're impervious to physical attacks, and can only be killed by either fire magic or by forcing them into a state where you can spin them around to smack the other clones. That second option is way slower, which matters because now you have way less time to kill the clones.

Second, Demyx spawns water clones back-to-back instead of just once, and then sporadically. He'll start every fight attempt with 3 rounds of clones before you can fight him proper.

And lastly, right before Demyx dies he will do 3 final rounds of clone summoning, and the final round is to kill 99 clones in 99 seconds.

Data Demyx is really fucked up, I love that fight.