This achievement is missable because there are no enemies after the story, and it only works in certain story sections. A good place to do this at is the second enemy encounter in the game, shortly after escaping the big cave in Cozumel (Mission: Hunter's Moon, Region: Cozumel, Objective: Get through the Dig Site). There are three enemies standing around a forklift. Equip the pistol and stand behind the single enemy near the bushes. Then, shoot them from right to left, all headshots within three seconds, to get the "Gunslinger" achievement. An autosave is created after killing them. Therefore, it is highly recommended to keep a separate save slot before this section. This allows you to reload it from the main menu, in case you fail and need to try again. In that case, make sure after reloading a save to immediately save in another slot again (the game always overwrites the selected slot with an autosave). If you fail in this section, you can also move on to the next objective. Lara will slide down to an enemy base where three enemies stand in front of her. You can restart the checkpoint in this area to try again.