Nothing but Net | Call of Duty Modern Warfare Trophy

Neutralize the 'Fog of War' machine gun with a frag grenade

During the first mission 'Fog of War,' the player ('Alex') and the marines were pinned by the train depot by a machine gunner.

If you keep moving forward, there will be a building on the left side with a window somewhat directly below the machine gunner. There will be a box of frag grenade you would use to throw at the machine gunner.