You can do this within minutes of starting a new game, no need to go through the Roxy missions.
At your first supply point equip the plasma rifle. When you get to the gate, run in far enough that robots start spawning in. You don't need to go far enough to activate the boss himself. Once the robots start spawning, get high, like on top of the walls. Wait for 2 robots to wander close enough to each other then take the headshot, using
to aim. Achievement popped for me on my first attempt.