First open the door with the stairs (not the slope), then buy the next 2 doors and buy Jugger-Nog.

Next go across the bridge to the side that you came from, next go forward and buy the door.

When in that room turn right into a room with 1 barrier.

There will be a red cup and green cylinder, go up to the red cup and hold X. It will make a grunting noise.

Now it will take 6-8 hits to go down.

NOTE: you can only do this once per game.