Cyber Akuma from Marvel Super Heroes vs. Street Fighter (level 8 difficulty)
He was almost unstoppable, not too hard to get to, but once I was there, it took rematch after rematch. I had to fight his cheapness with my own regular Akuma (dash dash, hurricane kick into air super hadoken)
Shin Akuma from Capcom vs. SNK 2 level 8 difficulty.
Not quite as tough as the MSHvSF cyber incarnation, but I couldn't kill him.
My brother finally killed him in a super close match, he used Haohmaru, appropriate demon killer.
Blaster Master anyone? The last boss was fairly tough, but the fact that you couldn't save progress, there was no passwords, there were limited continues...insane.
Back to the Future! timing in that game was insane, I don't think I ever got past the block kisses with your book level.
I thought Yunalesca from FFX was pretty hard until I figured out how the zombie stuff worked in the game.
Chrono Cross stuff...
Criosphinx is easy if you stole the yellow plate from the Earth Dragon. Put it on Glenn, and let him house on the Sphinx
You want hard? Try and beat Radius! In the very beginning in the training session! Sure he whoops you, and it's the first game right?
So we jump in there with the game/continue+ option. He still crushes us. He'll take more turns if necessary to rapidly cast enough blue spells to kill you. Then we discover the wonders of the color plates After going through the game twice (being roughly * level 96) and armed with blue plate.
Finally, we can take him down, and he can't stop us. Does he lose? Nooooooo.....he only comments that he must have trained you well!