At the end of the mission, you will have to walk up a hill where a sniper and another enemy are standing. The sniper is trying to shoot Vesper and he is complaining that she is moving too much. Kill both of these guys and pick up the sniper rifle. Clear out the people below you and then the people who appear to your right. Once you kill everyone, a helicopter should appear. Using your sniper rifle, make sure you have a clear shot on the pilot. When you have a clear shot, kill the pilot before the gunners in the helicopter and the helicopter will crash. Achievement unlocked.