This achievement requires you to kill 3 other enemies by shooting an exploding enemy.
Now further along in the game you'll encounter exploding enemies, which basically look like normal enemies but with smoke coming out of them. There are many opportunities to earn this throughout the game, but the best place I found was the first station you visit inside the tunnel on your way to the L-ABS Bunker in Act 2.
I made a game clip to show where in the level/how to do it. Just to note, I have an upgrade purchased in the settlement at the end of act 1 which is basically a laser sight, hence the line from my gun. If you don't have it, not to worry you really don't need it, it's just easier to line up a shot that way. Also I had already unlocked the achievement by the time I found this spot, so it doesn't pop in the clip, but you can see the enemy kills 3 others when it explodes.
If you miss the shot or mess up for whatever reason, don't worry. Just reload the checkpoint and you'll start off right outside the door beside that guy sitting on a bench (Still have no idea what he's doing there...)