Currently I'm trying Missile Boy. I figured out which levels are a problem to me, and with what character is a level easiest to get. In my opinion 6th level is not hard but I get very often killed so yeah...let's start with 6. Same thing with the 4th, 12th and 20th level. After beating these 4 levels you can start from level 1 passing the rest of the levels using these characters:
The Kid (1, 3, 6, 7, 9, 12, 13, 14, 16)
Meat Boy (2, 4, 18, 19, 20)
Captain Viridian (5, 15, 17)
Runman (8, 10)
Flywrench (11)
Why Kid, SMB and Viridian in those levels..you'll figure it out once you see the levels.
Why Runman...the fastest character in the game. You just need a little practice and the 8th and 10th level will become very easy. He is also useful for getting better times in some levels

And in the 11th level I used Flywrench because it's very easy to jump from the wall over the saws back to Bandage girl. You can try this one with the Kid, but I think it's easier with Flywrench

Good luck to anyone out there who is trying to get this achievement...I know I'll need it
