Not all steps have been found yet (at least by me). Add found steps in the comments, and i will update this thread.

Step 1: Teleport to Pack a punch, and grab the Summoning Key on the cart. (it may be in something that you need to blow up, i only noticed it after i noob tubed the room).

Step 2: Place the Summoning key next to the trapped keeper. Audio occurs, and the room near the keeper will open.

Step 3: Inside this room, there is Simon Says. The computers go from left to right, Red, Blue, Green, Yellow. If you have 2 or more people, have some of them train the zombies outside of the room, while you get time to yourself. The easiest way to do it is to write down the numbers that correspond with the colors, and then place the colors in order. You can have as many tries as possible. After 3 successful simon says games, the machine with the Wunderwaffe will reward it to you.

Step 4: (Credit to FlailingWombat for steps 4, 5, and 6)

shock 3 pilons around the map

1. inside Keeper room above door to electric Cherry
2. coming into power room from Cafeteria look straight and up a bit should be easily visible
3. up high on right side of linking teleporter behind spawn room

Step 5: you must "purge" 3 computers around the map

1. in centre of Juggernog room
2. in Tree room on centre table of box room
3. in electric Cherry room near centre of room

Step 6: Fill a soul box in the Keeper room

Step 7: (Credit to craigteacher)
After the box is filled, a debris will open next to the keeper, on the stairs. At the top, the buyable ending is unlocked for 40k points.