I'm surprised with the amount of people who aren't saying the Dark World Dr. Fetus. The first section is so frustrating because of the fact you have to wait about 10 seconds every time you die for the saws to start movie, and the second part has some of the trickiest jumps I've ever seen in the game.
I found the difficulty to be a natural extention of the going through the light world version. You only have to dodge two missiles before the saws start moving. After that you don't have time to worry about Fetus.
I actually had more trouble in light world because I was running in high detail and it wouldn't sync right. I only won because of a glitch where the saws pass through you and you can wait for fetus to catch up.