stairs to this room and get attacked by the sniper again, run

quickly to the other side of the room, take out a few enemies and go up to the roof. When you get to the roof, follow your command and watch out for snipers, creep to the area and wait to confirm the location of the enemy sniper, you will get a rocket launcher from your teammates, then get ready and wait for your teammates to cover the shot and then stand up and blow up the building opposite.