Bandage girl has a random chance to glitch every time you spawn on a level, and she can glitch in any level from 1 to 20 for each of the worlds 1 to 6. There is exactly one negative level for every world.
To unlock level -1 for a world, pick a level that is short and relatively easy. Below are good examples including a recommended character:
- 1-3 (The Gap) using anyone
- 2-10 (Johnny's Cage) using Jill (if you don't have Jill, see 0:48 in 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:
- 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.
- If she is glitching then touch her to unlock level -1. Otherwise, kill yourself, respawn, and go back to step 1.