Phantom Doctrine

As in the achievement description you have to modify any shotgun with a shotgun silencer.
You should get one in chapter 5 or later, mostly depends on luck . There are 3 ways to get shotgun silencer:
- buying after unlocking proper trade contract,
- crafting at the Workshop,
- finding at tactical mission in the containers.
First two you can unlock via solving secret documents, finding documents at tactical missions, agents interrogation or analysing signal at comms facility.
Any of this options force you to grind a little, or maybe more, if you have bad luck .

After you finally get silencer, you need an agent who has proficiency with any of shotgun model (yellow mark with star in upper right corner), equip him with this concrete model, and modify with shotgun silencer.
Achievement should pop up right after this.