Two players are required for this trick. Start the "Halo" level and continue down the path until you get to the part where you see a metal floor. Take a right up the icy path. Cortana should ask "Where are you going?" You should then see a bridge-like object. Go into the room. Take a left to find a terminal for the "Marathon Man" achievement. To the right you will see a ledge in the distance if you do not turn on your light (D-pad Up). Have a player with a rocket launcher stand behind the other player while the player without the rocket launcher runs up to the gap. The player with the rocket launcher must shoot when he is about to jump off. If timed correctly, the running player should fly to the ledge. Once you are on the ledge, jump to your left without falling or else you'll have to start all over again. Walk forward a distance to see someone dancing in his boxers with no shirt.