ThatOneBoss

[Arcana]

Oh, I hate that - when the game basically punishes you for the skill-path you took by making a certain enemy impossible. Example: I almost never used Aerith in FF7. Consequently she lagged behind on the levels. Got to the Temple of the Ancients, and lo and behold she is a mandatory character for ages. So basically Cloud and Barret were carrying Aerith (who got the shit knocked out of her every time...) until enough random battles had helped her catch up a bit. Then I fought...

Demon Gate in FFVII I hate that boss, took me a bit to beat it on my first playthrough x.x;

I honestly don't know how I forgot Demon Gate. That thing was nasty. Especially so considering you were still licking your wounds from the almost-as-bad Red Dragon, they hit you with this monstrosity. Thank God I had Barrier/MBarrier spells available...

FF7 actually had a few nasties, that were mandatory. The two mentioned above, then you had the Schizo twin-headed dragon and then that Dock-loader robot in the submarine base. Even knowing how to beat it I can still end up seeing the Game Over screen a few times against that robot...

[Caligo]

PE3 final boss on insane is hella difficult too. one hit one kill.

[Steahl]

I was sitting here trying to think about hard end bosses and...I have none. Sorry. I always wander around and end up insanely over-leveled by end of game, so bosses tend to be very, very easy. >.<