Super Meat Boy no-death achievements

Bandage girl has a random chance to glitch every time you spawn on a level. She can glitch in any level from 1 to 20 for each of the worlds 1 to 6, whether you're in the light or dark half of a world. There is exactly one negative level for every world.

To unlock level -1 for a world, pick a level in the world that is short and relatively easy. Below are good examples (in the light world) including a recommended character:

• 1-3 (The Gap) using anyone
• 2-10 (Johnny's Cage) using Jill (if you don't have Jill, see this video)
• 3-6 (The Shaft) using anyone
• 4-6 (Leviathan) using anyone
• 5-10 (Grinding Mill) using anyone
• 6-2 (Schism) using anyone

Then:

1. Look at or get to bandage girl and see if she is glitching. She will be spinning and making a distinctive pinging sound. I recommended levels where you can (mostly) see her from spawn.
2. If she is glitching then touch her to unlock level -1. Otherwise, kill yourself, respawn, and go back to step 1.
3. To play the level you just unlocked, go the light half of the world you unlocked it in. It will be available to play as a yellow-colored level beside level 1.

Unlike other characters, holding the Shift button (or the Y button for those with a gamepad) does not maker her run faster. In her case, it activates her special -- the color change and slowing of descent you already realized.

What it sounds like you haven't realized, is that when you let go of Shift, you'll perform an air dash, but become locked out of performing another slowfall / airdash until you touch another surface.

Something like this is rather hard to explain with words alone, so I direct you to this YouTube demonstration to clarify.