1) When you enter the selected level, press the button TAB in your keyboard twice, this will open a window that shows the stats of the level, this includes:
TIME
KILLS
And STYLE.
These three will display a rank on their side, (D, B, C, etc).

2) To get the P rank, all of those stats mentioned above need to get an S rank, not a B, not an A, an S rank.

IMPORTANT: You cant p-rank a level if you DIE ONCE.

3) Once you have it all done, go to the exit and you'll earn yourself a P rank.

So, you entered the level, this is what you have to do.

1) Complete the easy blue route FAST and with STYLE, do the same with the red hard route.
2) Once you have completed all the routes and placed the skulls in their places, a secret door behind you will open, BUT DON´T GO THERE YET!
3) Enter the blue route again and go into the first door in the right.
4) Aim at the ceiling with the shotgun, on those weird window squares.
5) Shoot and punch at the same time on the first window, it will create an explosion, you can use the rocket launcher too!
6) Go under the open window, jump and do a GROUND SLAM in the air and jump again as soon as you hit the floor.
7) Quickly enter, and then, you will see a bar of soap, grab it.
8) Enter the secret door at the start of the level.
9) Two SWORDSMACHINES will appear, just punch them with the bar of soap twice and they'll die.
Tip: If the soap falls, grab it again lol

FINAL STEP: Enter the boss room, and punch him with the bar of soap, once he dies, exit and finish the level.

THERE YOU GO! YOU EARNED A P RANK ON LIMBO'S MOST ANNOYING LEVEL!

So yeah, that's it, thanks for taking a peek at this guide, I'll take requests on how to p rank another level if you need it, anyways, bye!