The wall of choice is the one directly behind Skeram.
Walk up to the left side of this "wall" set autorun forward, and run into the wall so you are not moving side to side at all. Start at the left, and turn your camera SLIGHTLY to the right, so you are sliding along the wall as SLOW AS POSSIBLE.
You wont notice much happening at first, but you will start to push through the wall, suddenly your camera angle will change, to something like this. NOTE: THROUGHOUT ALL OF THIS, DO NOT, DO NOT, MOVE YOUR CAMERA ANYMORE OR YOU WILL POP BACK OUT. Once you get to this point, you are basically there, let your guy just keep running, and you will fall through.
This is where you land, a big open place with a big ass hole in the middle of it, go forward, and stay close to the hole. There are invisible prisons all around you, and if you go through one end, you cant get back out. WATCH OUT.
Follow the holes along the right side, until you get to the back. At the back you will see 3 jagged edges leading into the hole. This is the sweet spot.
Go to the farthest jagged edge (seen below) and face WEST on the very tip. Back up, and run off at full speed. DO NOT JUMP or you may overshoot and fall through the world.
Follow the ramp up, you will be outside Cthun's room. Take a peak at the openings of his room, and continue, around the corner. Suddenly, in a new room, you will see:
