This is done during Mission 1: Siege of Tobruk.
After a short introduction into the game, you will make your way to the mountain pass, and you will be asked to "Destroy the Nebelwerfers". There are 4 of these, and approximately 12 enemies in total in the area. Just stay crouched and work your way around the base. Sneak up behind enemies to stealth kill them, and use your Welrod in order to kill the crews operating the Nebelwerfers. Only shoot at them while they are generating noise.
If at any point they notice you, and the circle turns FULL RED, you have been spotted and should "Load Last Save". After destroying all 4 Nebelwerfers, and completing the mission, the achievement will unlock: