You can get this achievement as early as Mission 1. As you progress through the level you'll come across an objective in which you need to mount a turret that shoots explosive shells to destroy a plane blocking your path. When you destroy this plane, two soldiers come out. Blast them with the shells and then restart at the checkpoint. When you restart the checkpoint you'll start on the turret. Before destroying the plane this time, aim your sights at the plane and get off the turret. Hop back on and a third soldier will appear along with the other two after destroying the plane. Rinse and repeat this until you get to 50 kills.