Get 3 headshots with 1 magazine, using the secondary fire on the IvP-18 Tropov pistol

This is one of the easier weapon based trophies, which isn't saying much since none of them are actually very challenging.

The IvP-18 Tropov Pistol is the standard Pistol of the Helghast Army and is the default starting weapon of Hakha. It is not to be confused with the M4 Semi-Automatic Pistol. These two weapons look almost identical, and their two most distinguishing features is the burst fire mode of the Tropov, and the lack of secondary fire on the M4. The IvP-18 Tropov Pistol also has a larger magazine compared to the the M4 Semi-Automatic Pistol.

Moving on, the secondary fire on the IVP-18 Tropov Pistol uses a semi automatic mechanism making it much more accurate. A fully loaded magazine of the Tropov Pistol comes with 18 bullets, and you need only connect three headshots; hence a little bit of indiscretion is permitted. On Easy difficulty, one shot to the head from the Pistol is enough to kill a regular Helghast soldier. Make sure you do not reload your weapon until you have earned yourself three headshots, which needn't be consecutive.