I have to say that the only hard RPG bosses are those on something like Chono Cross where you can't just simply level up to kick ass... I'm working on a game myself that it takes longer for you to level up so you'd be more encouraged to do side quests and finding secret weapons and such to power your character over beating on the monsters (which are strong anyway)
hard bosses on an RPG would be something like Olga Flow from PSO, or just annoying like Spikey on Secret of Mana
but honestly, it would be something like one of the senior dragons on Dragon Seeds, where you can only train so much before your dragon just becomes too old to fight, then you've gotta have SKILL, not just whacking endless monsters
hard bosses on other genre games would include Biolizzard from SA2b, Meta Ridley from Metriod Prime (holy shit! how much punishment can he take? and he's never open for a shot when he's almost dead!)
hmm... what else...I know Dario is IMPOSSIBLE without going through the game the second time, poora poora Riddel...
and if you want a super uber hard badass, play Star Ocean 2, goto the VERY end, save at the last save point, and go back to the first town you arrive at on the second world, talk to the girl that appears there, now the boss is so strong that you'll crap your pants! ... the only way to TRY to counter his strength is going to the old man in the arena, talking to him, going back to the first planet, and jumping on your bird to the hidden dungeon! you better not beat it until you REALLY think you're ready to (somewhere over level 250...) and make sure you pick up Ashton's triAce attack! it's a must!
