This is during the mission "Victor Charlie". You'll reach a point in the level where you are told to pick up the Grim Reaper and you'll need to use it on a target across the water.
Keep hold of the Grim Reaper after this as this is not the MG emplacement required for the achievement.
Proceed through a building with Woods until there is a slight up hill towards a house. Next to the house is an MG emplacement.
Use the Grim Reaper to take this one out and the achievement is yours.