On the Marines side of "Flag B" is sheet metal guarding the top of a building. Climb the ladder to reach the top. To stand on the sheet metal itself, climb onto a ledge and slowly go back and forth until you see the "Climb" option. Proceed to the ledge besides the building overlooking the tiny enclosure. Jump on the white radiator you come across. Notice the little knobs sticking out of the buildings and carefully jump on each knob until you can jump onto a balcony. From here you can snipe 180 degrees from the crumbled building to the construction building.