How do I kill tis damn witch without startling her. What's the trick?

You ignore her. Turn off your flashlight and don't get too close. It's not about killng her. If you absolutely feel you must, chuck a molotov cocktail at her and fill her with bullets and she will drop fast.

It is possible to headshot her, as there's an achievement for doing so. I went up to her with my flashlight off and unloaded a shotgun in her face as she was standing up.

its called cr0wning the witch. Get an auto shotty, walk right next to her, aim at her head, and unload as fast as you can. If you do it right, it'll only take one shot.

You don't, you simply avoid her if you don't want her to activate.

Avoid here is best. If your trying to kill her for the achievement autoshotgun is your best bet, just wait until she stands up and go to town. You will not get a lot of shots so do it on Easy. Your other option is molotov her someplace where she can not really get to you but that is a bit more dependent on where she is.

This.  Auto shotguns ftw.

no wat u do is u go up and pucnh it in the back jk.wat u should do is have all 4 teammates unload while u through a molotov that should get her really quickly

Yep, but it takes her a second. She'll look up at you first like "Whut?", and that's your chance to blast her.