This may take multiple attempts, because a lot depends on luck and timing. It is not simply getting a headshot. It's a single killing headshot.
You must approach the witch quietly, flashlight off. I recommend the auto shottie for this. Aiming with the hunting rifle will take too long. You cannot snipe from a distance. You must get her in the moment she is startled. Turn the light on, she'll turn her head and her eyes will glow - THAT is when you fire. Once she leaps up it will be too late. You have to be quick.
It all happens extremely fast so that's why I say luck and timing.
Recommend level 2 chapter 4 (the Town) as you come down from the tower - a witch is sometimes (random) in the alley behind the first building. Close enough to the safe room to reload and try again if she isn't there. There is another witch in the office building later on in the same chapter; the one you have to jump up on the car, to the ledge, over the rail, into the window...
Keep trying you WILL get it.