I just recently beat the challenge of the gods in God of War. The goal of the last stage is to raise a platform high enough so that you can jump up on to another platform, which is your goal. Get there, and the level's over. Fall off the platform or get hurt too much, and you die.
To raise the platform, you've gotta kill the enemies that spawn on it. Any time you kill one, another spawns to take its place, so there's always three little mini-cerebrus dogs, and two of these nasty bastards with big pointy staves. The guys with staves can reach well over 3/4 of the platform with their attacks and have some of the most ridiculously long, ground-covering combo maneuvers I've ever seen. The dogs have the annoying tendency to just bop you off the edge of the platform when you were just about to attack an enemy.
You can't block for any significant period of time or you'll get knocked off the edge. You can't get hit while you're in the air or you'll get knocked way off the edge. You get hit once, and it's almost a complete certainty that you'll get hit again until you start blocking (taking no damage but still you can get knocked off the edge) or jump (taking the risk of getting hit in the air and falling off immediately). The enemies are all basically attacking constantly, so finding an opening is a real pain.
I know it's not technically a boss, but that level was definitely this-thread-worthy. I sat there for over two hours straight (at around 30 seconds an attempt) and just died, over and over again, swearing constantly at how retardedly hard the level was. Finally beat it with some random combination of insane luck and awesomeness.