Kingdom Hearts has a few bosses like this (I'm going to agree with a previous poster and say that Demyx and Xaldin can fuck right off), but I've got one better in terms of being annoying and time-consuming.
In Kingdom Hearts 358/2 Days, there's an optional boss in one of the last missions, basically the hardest boss in the game... sort of. See, he's just a pallet-swap of the normal dragon-type enemies, but with CRAZY high stats, RIDICULOUSLY high HP, and a freaking annoying ability: Every single attack of his causes a random status ailment. EVERY attack he has.
What this boils down to is this: You have to PERFECTLY memorize all of his attack patterns, because if he hits you with a SINGLE one, it could cause an entirely debilitating status ailment, that'll basically be an automatic death sentence if you're unlucky. His hit points are so high, that it just drags and drags and drags the fight out, for no particular reason at all.
It's really more of an endurance match than anything else. Seeing how long you can continuously fight the same annoying enemy without making a SINGLE mistake. He's not really hard in that respect, his attacks aren't particularly difficult to avoid, it's just that after fighting him for SO long, you're bound to make a mistake sooner or later.
So, while there are TOUGHER enemies in the Kingdom Hearts games, this guy wins in terms of being both annoying AND time-consuming.