Heartless:Large Bodies and their Agrabah lookalikes used to bug me, but I found a trick for them: Switch to Valor. That skill when you hit Square mid-combo: just attack once (attack is deflected) and then nail Square, and the attacks
always sends them flying up/back a bit, giving an opening for a combo.
(Well, of course there's also the Reaction Command for the Large Bodies, but that's only when they slide around anyway, so although this trick is aimed more at the Agrabah ones, it should work fine for the originals as well.)
My god. Not so much in the easier modes, but in Proud Mode, any of those magic-casters were annoying as
hell. They'd always dodge half of your attacks. (I might be thinking of the first game, but I think they were similar in KH2.)
On that note, yeah, those bigger fire-based ones... ugh. Enemies with uninterruptible attacks are epic failure. If it weren't for that, they wouldn't be nearly as much of a problem. If it's just one, or they're close together, Blizzard spells work well. (Also stuns them, if I remember correctly.) Otherwise, I just rely on physical attacks, and take the resulting beating. As long as I hurt them more, I don't care. Lol.
The monkeys occasionally bugged me, but no big deal ultimately. (Mainly when they keep teasing you, jumping out of the way of attacks at the last minute, constantly. Happens more in Proud.)
Those jack-in-the-box things were indeed annoying. Both of them were, really. The claw-spamming ones, and the ones with guns. I don't even remember exactly how they were annoying. I just remember them being annoying.
Gummi Ship-only -- Those cling-on things always annoyed me. Even if you just spin and send them flying, they'd die, but you wouldn't get any items from them. I don't think it even added onto your score. If you didn't kill them quickly enough, they just got annoying. ...Was always fun to send a bunch flying off at once, though, and use them as weapons. ^^;
Nobodies:Dancers, period. ...Well, almost. Dancers, I think, annoyed pretty much everybody. Anytime I see them, I tend to just switch into some form or other and spam... whatever has a good change of wiping out as many as possible in as short a time as possible. (Usually Firaga/Thundaga, though Wisdom's long-range attacks help, and Master/Final's hovering blades help a ton.) Again, enemies with uninterruptible attacks are epic failure. They just come out of f***ing
nowhere and grab you, and once they do, there's no avoiding it whatsoever. Sure enough, they tend to chain them together, too. As far as I know, there's no real way to dodge it, either, other than jumping and praying.
Berserkers, though my main issue with them is when they spam that pogo-stick attack. They're practically invincible during it, and when they keep chaining them, it gets horribly annoying very quickly. Thundaga-spam is the way to go with these guys. (God bless Master/Final forms.)
Other:The undead pirates. All of them. When Port Royal's got more shadows than moonlight, it's pretty annoying to wait for their weak point to appear.
Bosses:The Grim Reaper, in Port Royal. Just like the pirates, temporary invulnerability is a pain. Hey, anyone ever gotten screwed by getting Anti-Form in this fight?
Wonderful. No Reaction Commands, and thus, no progress.
Demyx, 2nd fight, in Hollow Bastion/Radiant Garden. Specifically, in Proud Mode. How long was it you had to get rid of the clones? Like, ten seconds? Ridiculous. And if you didn't, you got a game over? Why? That never even made sense to me. "Oh god, I've been swarmed to death." *sigh* Anyway, yeah, Demyx was somewhat of a challenge in his second fight (despite Jiminy saying he can't fight well), but especially in Proud Mode.
Xaldin. Honestly, I didn't have that hard of a time with him at all in my first file, while I still had problems with everyone else I mentioned. Once I jumped into Proud Mode, though... Man. I got my ass whooped by him so many times. I was lucky enough to get Mickey a few times, but he didn't help. He probably died even quicker than Sora did. I just kept retrying until I finally got him with Sora.
Gummi Ship-only -- My god. Has anyone else fought that thing at the end of Assault of the Dreadnought Level 3? Whyyyyyyyyyyyyy? I finally took it down, but man, that battle lasted a good hour or so, I think. (Maybe it just seemed longer than it was.)