CR0WND Achievement (L4D)

So.. I decided I'd take a look at the different Achievements, and I came across a funny one, which I thought was impossible - CR0WND - One-shot the witch. G'damn, I thought. But yeah, I figured I'd give it a try.

First order of business, the hunter rifle - what better than a rifle with a scope to take out something in one shot? Well I was wrong. It failed.

Obviously the only other choice was the auto shotgun. It didn't quite do the trick most of the time, but I did manage to get it! And well, I realized later I should have attempted to do this on easy, instead of normal - I'm fairly sure it is impossible on advanced and expert, but if you want to give it a try, be my guest, and do tell me how it goes.

So..
In order to get the CR0WND Achievement - Use the Autoshotgun, on easy, get as close to her as you can, and blow her head off.