Can only be done in single player.
This will come naturally over time, and - due to the nature of the game - depends on luck as much as skill for how early you'll unlock it.
For me, it was the first achievement I unlocked. (I find the challenge quite substantial, and unlocked it before even beating the first boss in Level 5).
My in-game clock timer, found on the large display in the hub room where you upgrade your stats, sits at 1hr 30mins, so it's definitely an early one. There isn't any need to grind at all.
Tip:
Scour the levels for every last enemy before continuing. Many levels have a timer which activates at the top of the screen after about two or three minutes, counting down until reinforcements arrive. Stand by the elevator to the next level, and aim your gun at the door. Around 20-30 guards will show up, make a bee-line fore your position, and can be taken down en-mass with either a shotgun, or the laser-burst skill as they swarm into the room. This also give you a chance for health / special / credit pick ups too.