In Forge mode, get on a Mongoose or Warthog and circle around the map until you find a corner with a window. There are two ways of doing this: the first way is to forge a crate or something that can reach the window. Place a machine gun turret on top of it, making sure the turret is close to the window. Then, use the turret and look up. Turn into the Monitor to leave the boundary. The second way is to have a partner forge a turret and set it down. Have the other player use it, then have your partner pick up the turret and push it against the window. The other player should look up and turn into the Monitor.