This is easy to obtain using a shotgun since the idea of this weapon it to have a large blast radius. If you run up close and shoot without aiming down the sights, you will likely kill the enemy with 1 shot if they are not armored or anything. The pistol is quite accurate when firing from the hip if you fire slowly. Using a machine gun is possible at close range, but the kickback makes it highly innaccurate.
Defeat 20 enemies firing from-the-hip