Here's the basic meatboy dark ending route.

Do these stages: Forest: 1-19 (both light & dark) Hospital: 1-13, 16-19 Factory: 1-11, 14-19 Hell: 1, 3-17, 19 (easier) or 1-17 (faster with skip in 4-2x) Light Rapture: 1-7, 9-18 Dark Rapture: 1-2, 4-7, 10-18 End: 1-5, light fetus, dark fetus

You can skip 2 stages in dark rapture because you did extra stages earlier (19 in forest instead of the 17 you need). This route skips 5-3x and 9x, for reasons that will be very obvious if you try them yourself. You can also do more stages earlier (1-20, 2-20, etc) if you want to skip more. It's also worth noting doing 5-3x (and skipping 1-19 and 19x in forest in exchange) is actually a couple seconds faster with optimal strats, but this is not recommended for most players.

To perform the skip in 5-1x: watch a run to see where to set it up. Get in the corner next to the saw & let go of run (don't hit it again until you're through with the skip). Hold left & hit jump twice really fast, & hold jump on the second press. Keep holding jump & left and you should make the skip. The timing is pretty precise, so if you just barely clip the ceiling, keep trying.

For each chapter, do the light world, then the dark world, then the boss. Since glitch girl only starts spawning after the boss is cleared, you never have to worry about her.

One last note: This category fulfills the normal requirements to unlock dark fetus, which are to complete 85 stages in the dark world and beat light fetus. There's a glitch that lets you skip to dark fetus after beating 6-5. It's not hard to do; just mash Y to change to the dark world immediately when you return to the map after 6-5, and the game unlocks dark instead of light fetus. This glitch is not used, because if it was, the run would just be any% with dark instead of light fetus (and then the game would crash after the credits).