To leave the level boundary in the level where you start outside and walk over a bridge that you must walk down, continue up to the area with the elevator. After entering the elevator and then out underground, go towards that thing that can press to make the hole digger appear and kill all enemies. Turn left to see a tunnel where the hole digger that goes down and then up. Enter it through the other side; there will be three enemies on your left. Kill them and go to where they came from. There will be a dead end on one side and the passage you are supposed to take. While facing the dead end, walk up the right side of the dead end by pushing UP + RIGHT. Continue to walk up and you will be pushed out of the level boundary. Note - This only works when you have certain guns equipped.