This is semi-easy if you have a team backing you up. Begin by heading up the stairs and placing the team on the landing so they can fire both at the floor level and the top of the roof. Head around the corner (to shield yourself from enemies shooting from the stairs) and fire a few rounds at the rappel point to draw enemies out. Raise up the tac-map (won't work in split screen) to make sure enemies do not appear in the distance at Building B. Your team should be able to guard your back while you hose the suckers coming down the rope.