Demyx is just like his element: water. It looks weak and harmless, but push him the wrong way and you get a tsunami. It's not really that he's a terrible fighter, at least, not in my opinion, it's just that he doesn't enjoy fighting, so he avoids it whenever possible. SDG calling him a wimp with no heart, and the thought that Roxas had betrayed him, was enough to set him off.
The Water Clones are probably the most difficult part, but I think he's challenging enough without them, depending on the difficulty setting. It's because he moves so fast that it's hard for me to trap him in a good combo, and he never sits still long enough for you to just keep whacking him with the Keyblade. Those geysers are annoying too.