MUAAHAHAHAHAHAHAH!
Give Touhou a try.
Look up these bosses, and I assure you they are THE HARDEST BOSSES IN HISTORY.
Touhou 6: Flandre
Touhou 7: Yukari
Touhou 8: Mokou (this game got me curious about the whole series)
Touhou 10: Suwako (pretty cool patterns)
And....I'd say my second favorite Touhou 11: Koishi.
Yukari has to be one of the hardest in history, but try Mokou.
I have yet to play the real game, but the demo was so addictive.
11 just came out, and I was fortunate enough to find a video of the extra stage boss for it.
Right now I'm working on playing all of their songs on piano, so that makes them even HARDER (they have some pretty complex themes).
Other than that....
KH Seph wasn't hard.
I was kicking his butt at 12 years old.
YAWN.
People were right about FFX.
Haven't played it, but I've seen vids.
E-Z.
FFXII: Crap, I can't remember what the thing's name was, but it had like 50 million HP.
Ya...yaz....something that starts with a Y.
I think.
Almost any game with an optional boss generally has that thing set aside as more of a challenge than the last boss.
I plan to do that if I ever make a game.
I'm talking OVER 9000! HP.
Seriously, I would have it at around 900,000,000,000 (that's 900 billion).
Give the player a while to beat that, say a couple days.
Of course, there is saving in the battle, let's not make things too hard
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.
Oh, and try FF:CC WITHOUT casting Holy on the Ghost Behemoth.
Good luck, it took me forever to defeat it.
Talk about a hit and run, and I was in the 5th grade.
Boost Guardian of Metroid Prime 2 is a pain (without power bombs, you speedrunners).