On the mission 'Halo', get 3 of your friends to play with you via Xbox Live, turn the difficulty to legendary and turn on the iron skull (note that if you complete the level in this setup you will also get an achievement). Complete the level up to the part just after you beat the oracle, once you leave the inside of the structure, kill the two sentinels and then turn to the left and head down the slope. Then have a look underneath it and there should be four ghosts, get everyone to take one and then take them back up the slope. Continue with the level until you get inside the structure that seargent johnson came from. Take your ghosts in, hop out of the ghosts and kill everyone. You should get a checkpoint after killing everyone (note that all four players must be in the room for this to work). Now go back to the door in which you got in from. It should be locked, if it isn't commit suicide and you all will respawn and it will be shut. Now go to the door that you are meant to go through, instead of the door being there, instead the wall will show the view of one of the four players. If it isn't there keep killing yourselves until it does happen. It may take a while, it only happened to me by luck.