To get the achievement, you need to start the "
The Command Spire" mission and
defeat 40 enemies with a Stalker Rifle. You can get one in the mission, but it is advisable to have one by launching the mission to unlock the achievement more easily. Perform precision kills to save ammo. When you run out of ammo, don't drop your gun, reload it.
Tip: Activate the
Bandana Skull to have infinite ammo on your Stalker Rifle.
Here is a video guide (in french) to help you: