How about undead that's hard to kill? Mabinogi is a perfect example. The zombies, unless you're really good with ONE skill, they take forever to kill. The best part about is, unlike every other monster/player in the game, the can walk while using the Counter skill, and stand still while using Defense instead of walking. All attacks do 1 damage and have no knockback except 2, Smash, which won't work if they're using counter, and Windmill, which is weaker if they are defending and takes away 10% of your max HP. Did I mention that it's nearly impossible to tell whether they are defending or countering? Did I also mention that you HAVE to kill 50 of them for a quest to revive in town, or else when you die, you have to go back to the other world or wait until some random passer by revives you? (The zombies only appear in "Another World", which you can only get to with a special pass and even then, you can only enter on Saturdays)
So I'm adding these to the list:
Enemies that can use the same attack/move as every other monster, but they can do something else with it.
Enemies that have skills that are nearly impossible to tell apart, and the 2 skills make a huge difference in the fight.
Fighting an enemy that is easiest to kill with the one skill you never use/upgrade.
How about when you BOTH hit eachother, but it doesn't count for you, because the boss has some dramatic death sequence before he's officially dead. I forgot what game it was, but this has happened to me a few times on a game. I think it might have been Megaman.