Resistance 3

This trophy is self explanatory, and will likely unlock naturally as you play through the game. Whenever an opportunity presents itself, you should always try for headshots, as they kill enemies much easier and faster. As long as you try for headshots, this trophy will unlock very early in the game. The best weapons to get headshots with are the Deadeye (upgraded version shows headshot hitbox), and the Marksman, because they can zoom in and provide increased accuracy.

If you, for some reason, don't get 50 headshot kills before the end of the game, you can continue the progress on another playthrough, or through chapter select. Like all other combat trophies, it can also be farmed at a checkpoint, but it won't be needed. You can monitor your progress through the Rewards section of the Extras menu.