On the Marines side of "Flag B" is sheet metal guarding the top of a building. Climb the ladder to get to 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. Make your way to the ledge besides the building overlooking the tiny enclosure. You will see a white radiator. Jump into it. There are little knobs sticking out of the buildings that look like this.
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Radiator>[l] l l l l
Cautiously jump on each knob until you can jump onto a balcony. From here you can snipe 180 degrees from the destroyed building to the construction building.