This is on co-op only.

Go by the help door and the stair case that in the spawn room.

One of you go up stairs and go to the room with the bar.

From there go to the first window and there will be a plank of woood and what looks like a muddy line.

Stand directly on it, but lean against the wall and everyone else then needs to stand at the mystery box.

Everyone should be invincible. Send a game invite if you need help.

[unconfirmed]