Bulletstorm: Full Clip Edition

An easy place to get this achievement is just after you first get the leash. You're taken to an area where you have to pull enemies onto a wall of bent iron.
You can just take cover there and headshot all the enemies on the hill, you seem to have unlimited ammo at that point, and the enemies also keep coming until you pull them towards you with the leash. Don't use your leash to drag enemies towards you until you want to progress with the story.
I've added a screenshot of the area I'm talking about, hopefully it works.

Note: This is just after collecting the fusion core for Ishi, breaking through the wreckage where the game first directs you to pull two enemies towards you (these two enemies can only be killed by the leash)

Hope this makes sense, this is my first small guide!

I got this before getting the leash.

If you've already completed the game, you'll have to play from the prologue and move through from there.
Starting at 1:1 will.only hand you three headshots.

It's easier to do this on very easy and aim for the head with every kill before getting the leash. This way, you can kill jow you want after getting it and pick up lots of skill points early on.