Hint: Lobby: Secret roof area
Go to the second level of the Lobby multi-player level and grab the rocket launcher. Walk over to the edge of the balcony and start shooting up towards the wall facing the lobby itself. After enough shots you will dig a tunnel to the roof. Walk through and jump right up. You can shoot out the glass and snipe people down on the second level. There also is a Fusion Rocket Launcher that you can grab.