Xaldin was hard, I didn't really find Demyx hard unless I didn't feel like using the Water Dance thingy or whatever it's called (it's been a while), I found Xaldin so bad that I couldn't land but a couple of hits on him as Mickey if I happened to be late on using a potion or curaga...
But I think Xigbar beats them both. I mean, I came back to fighting Xaldin, and it's pretty easy if you do a lot of jumping and have a lot of potions or whatevs.
But then again, it's different for everybody...