Kill 10 Helghast with the M224 Mounted Machine Gun

There are a couple of places where you can unlock this trophy. Just remember that kills stack, so after clearing the place of all enemies, you can either restart from the previous checkpoint, kill yourself with a grenade or choose to continue playing. Either way, you will probably earn this trophy over the natural course of the game unless you go out of your way to avoid mounting the Machine Guns.

Chapter: Helghast Assault
Level # 4

After you survive the ambush on your unit, you will head through a gate and into a building. Three Helghast soldiers await you. After killing them, head upstairs, and you will see your first M224 Mounted Machine Gun. You can kill at most 9 soldiers here, providing your partner doesn't kill one.