Xaldin is very hard, especially facing him the first time, was hard because it was one of the fights where button mashing wasn't exactly going to get you far. For me, every time I tried to run up to him, he would hit me with his lances before I even got to him. And he can combo and hit you relentlessly before you can even heal, making it even hard when Mickey comes to save you. Though it's kind of easy when you time your attacks right, use the Jump reaction command well, and get the hang of it.

Demyx was also very hard. You could pull off a few combos on him, and then he counters and knocks you back with his own. Definitely the hardest would be when you have to kill the 10 water clones in 10 seconds. Even with the Wild Dance reaction command, it was VERY hard, and this fight could arguably be the hardest battle in KH2 (not counting Sephy).

A lot of people said the have trouble with Xigbar too, though he wasn't THAT hard. Well sure, he does relentlessly shoot lasers at you and only stops every once in a while to reload (since when did you need to reload a laser gun by the way?). Again, this is one of the fights where running up and X mashing probably won't get you very far, since he could just attack you from a distance and teleport before you even got to him. But really, its kind of really easy just to Guard and deflect his lasers right back at him and use the Warp Snipe reaction command.

So, in my opinion, it's either Demyx or Xaldin that's the hardest.