To get this Achievement you must finish a Weapon Drill & 2 easy guns for this are the Needler or Rocket Launcher.
The Weapon Drills section can be found in the Academy Section at the main menu.
You can also earn the Deadeye Achievements alongside this one if you earn 3 Stars in one Drill.
After earning these you could try earning the Augmented Achievement.
For the Augmented Achievement you must finish 5 Weapon Drills with 3 star ratings.
Some people say that the Augmented Achievement can be unlocked by playing 5 different challenges instead of different guns if each of these are finished at 3 stars.