At the point where you must rescue Sergeant Johnson and his team, snipe as many enemies as you can and then jump down and cross the bridge.
Keep to the right of the bridge until you see a blocked line with some pipes. Hop onto this block and continue.
When you reach the end of this raised block, jump up into the balcony.
Follow this balcony to the end. The skull is hidden behind a pillar jutting out from the building tucked into the corner.