Step 1: collect all four fuses
1- in the library close to quick revive
2- in mule kick room next to the Mog 12 wallbuy
3- across from the spitfire wallbuy on the cabinet
4- continue from the spitfire and open door to the right and it is on a wheelchair

Step 2: insert fuses at the machine after the 2000 point door door and turn on power

Step 3: find the summoning key at three locations and knife it(in order)
1- in the spitfire wallbuy room
2- next to quick revive
3- to the left of the escargot wallbuy

Step 4: fill the three soul orbs around the map in 3 locations in no particular order( will give you weapons after each one is filled):
1- escargot room
2- mule kick room
3- quick revive room

Step 5: go to the power room and the summoning key will be on the floor. Knife it to break the wall and obtain the hells retriever

Step 6: go to the library, pick up the mk2, and shoot 5 glowing orbs around the map
1- on close tree by quick revive
2- on wheelchair in the mule kick room
3- in the barrier next to escargot wallbuy
4- in the power room on the end of the electic pole hanging from ceiling
5- in the jugg room on banister on the right side of stairs

Step 7: play the pianos(in order) and unlock the portal
1- in the hallway between double tap and escargot room
2- in the room across from library
3- in jugg room next to portal

Step 8: go to the quick revive room and an altar will rise out of the ground and the summoning key will land there. you will then be able to distort the world. it will take one round to happen. you can un-distort the world by interacting again.

At this point, you can distort and un-distort the world between PAP and perks. You can only have one or the other at a time, and it takes a round to flip between them.

Step 8.5: go into the showers and interact with the stone across from stamin up in the clinic's showers. This unlocks step 9.

Step 9: link the worlds by interacting with 4 stones in both the distorted world and the regular world.
1- in the quick revive room, across from the RK-7 wallbuy, covered by vines
2- in the clinic of evil near the portal
3- in the room between power room and jug room
4- in the speed cola room

Warning: you cannot go back and pack or buy perks if you go beyond this point.

Step 10: Shoot 3 flaming, floating rocks located outside the map at:
1- outside of the quick revive room, opposite of the mystery box
2- above the portal to the clinic, looking out of the window
3- out of the window opposite of the Mule Kick machine, it will be slightly to the left

Boss Fight time.

After shooting the three rocks, everyone in the game must head back to the quick revive room, and someone needs to interact with the pedestal. After that, provided you are in the corrupted world, you will all be locked into the room, and you must end the round. When the round flips, a big ol' orb of doom and gloom will spawn. Shoot at it until it dies. Max ammos will spawn in the center at your convenience. Magmagat is recommended, as well as a gun with lots of ammo. After it dies, you can interact with the pedestal again to end the game.

Found by A Tiny Russian Drunkard, MrT_Plays, and DJ
If you have any questions or issues, comment and I will update the guide or point you in the right direction as soon as I see it.