Left 4 Dead

You can also see CrOwned because it is possible to pop both achievements at once.
This is much easier to do than CrOwned. All you need do is kill the witch without anyone taking damage from her. She is most vulnerable at the moment of being startled - when she'll turn her head and eyes glow - just before she begins to rise.
Get as close as possible with your light off and use the auto shottie. But don't fire until she is aware of you. Turning your light on will startle her - then pump away and back pedal.
I got this (but failed the CrOwned) when she leaped up and I backed up and kept firing. Oddly, she did actually hit me and caused slight damage, yet the achievement still popped.
Recommend level 2 chapter 4 (the Town) - the safe house has an auto shottie. Climb down the tower and a witch is sometimes (random) behind the first building in an alley. If she's not there it's easy to reload and try again. (Also if you fail.)
There's another witch further ahead on that chapter in the office building - the one you have to leap up on a car to reach a ledge, then over a railing and into the window.

Most people get this alongside Cr0wned. To do this, you have to run up to a witch with an auto shotgun and aim for her shoulder / chest area. Simply shoot as she starts coming up, it doesn't matter how many times you hit her as long as she doesn't do damage to anybody on your team. Some people prefer to do this behind her, others in front of her. See what works best for you!

Hope this helps.

Witch Hunter - 20G (Original Achievement)

Difficulty: 4/10

The best way to do this is to CrOwn the witch. In other words kill her instantly with a head shot with a Shotgun. If you have never CrOwned before, it will be a very frustrating and difficult thing to do, but you will want to practice as there is a different achievement that requires you to CrOwn a witch.

Another way to get this is to have one person get ahead of the witch and shoot her, that person then runs to the safe room and shuts the door. From here the witch will panic and run away. The other 3 people should be shooting her the whole time from when their teammate originally shot her, until she hits the floor.