The final boss of the Conan game. He's not so much "hard" as he is "obnoxiously over-drawn and irritating."
Also, the final bosses of almost every fighting game--especially the following: Tekken 6, any Mortal Kombat after 1, Dead or Alive 3, Tekken 5, Mace: The Dark Age, etc.
These bosses aren't so much challenging as they are cheap and programmed outside the set rules of the game they're in. The final boss should be challenging, but it shouldn't be operating outside the norm of the game what with killing in three hits (Tekken 5, Mortal Kombat whatever), being stupidly oversized (Tekken 6), or forced to fight from an awkward angle (Dead or Alive 3). Honestly now, if I'm fighting something that takes up 60% of the screen and my hits only ever reach his knees, that's just bad design for a fighting game. Eyedol was one of the best final bosses I've ever seen in a fighting game because he was powerful, impressive, but fought within the rules of the fighting game--it was fair.
For the issues I had with Seth in Street Fighter IV--at least he largely operates within the rules of the fighting.
