Perform 3 headshots in 3 seconds with the pistol.
This trophy is missable. You can probably achieve this trophy during the second enemy encounter in Cozumel. There are 3 enemies standing around a forklift - equip the pistol and stand behind the single enemy near the bushes. Shoot them from right to left.
It creates an autosave after killing them, so if you fail, it might be wise to create a separate save slot to try again.