First, do the "Run while floating on wall glitch in Multi-player" glitch. Once you are there, walk out of the corridor and face toward the pipes. On the left you will see a corner with a chunk missing out of it. Have one player walk to the corner while facing it, then walk into the corner slowly while hanging a little to the right. If done correctly, the other player will see your leg slide into the wall. Once your leg is in, move your torso around to get your blaster arm to also disappear into the wall. Note: Do not move in any direction but just pivot. This glitch is difficult to do.