We Happy Few

All you need to do is get 100 Headshots. They don’t need to be headshot kills. Below is a video that shows an easy grinding spot. If the enemies stop spawning for you then jump off the edge to respawn and then the enemies should start spawning again

After that room with the shootable buttons and switches on 3 columns you have a cutscenes where you have to run away from the robots when you see the first barrel on your right, jump on the hand railing and carefully walk a bit further on it, then jump on the stacked box and your out of reach from the robots and killing them makes new ones spawn.

I noticed that robots that fall to their death do not get replaced by new spawns. Also if robots get bored they walk to the far end of the walkway, so pay attention to let the spawning stop. I ended up with 2 robots that i kept close and respawning till the achievement popped up on my second playthrough