1) Load the mission "Ring of Steel" on either solo or co-op campaign.
2) Play through the mission until you exit the asylum.
3) When you leave the asylum you will see two tanks.
4) Go to the tank on the right.
5) Hold "X" next to the tank to enter the turret.
6) Spray the turret at the German infantry soldiers and try to kill as many as possible before the screen blacks out.
7) If you managed to kill fifteen soldiers before the end of the mission, when the screen blacks out, the achievement will pop up.

Here is a video tutorial: