When the witch is still sitting on the ground crying her little heart out, you turn your light off and simply walk in behind her while keeping the left analog stick pushed forward.

She will stand up but she is unable to hit you because you are pushing against her back, then you open fire to stop a witch dead in her tracks.

Note: if she is already standing then you cant do it., she has to be sitting on the ground and you have to approach her from the back, not the front!