The easiest location to get this achievement is early on in
Act III Chapter 4: Do Not Go Gentle.
At the beginning of the chapter you will follow a path before opening the first door. You will encounter a group of enemies and after clearing them out (it is recommended to not shoot any pods in this room to avoid Juvies spawning) you will continue on to see a Juvie drop from the roof.
Continue along this path and you will drop through a hole in the floor, at this point
a Buzzkill will be directly in front of you on a table, pick this up and bring it to the left, bringing it up some stairs and turning to the right you will find a hallway filled with Pods.
To use the Buzzkill hold down
and press
, you should aim for the pods on the ground, as popping them will release a Screamer, which will pop all the Pods in the vicinity causing more enemies to spawn, once enough enemies are filling the hallway (or if they are getting too close to you) shooting the Buzzkill blades down the Hallway should net you the achievement as the large amount of Juvies often line up, and the blades ricochet off the back wall.
In 4 attempts I was able to kill 3 with a single blade 3 times, obviously Casual is recommended, but this can be done on any difficulty.
If the achievement doesn't pop simply restart the checkpoint, you will have to clear the previous room of enemies again, but it can be done very quickly and shouldn't take you many attempts.
From the beginning of the chapter I was able to get this achievement in just a couple of minutes.
-------------------------------Alternatively if you don't like the detailed guide, I also made a quick video showing how it is done: