There's the legitimate way of getting this, then there's various glitches you can use if you don't want to get it in a noble fashion.
Legit: I would recommend playing a campaign you know well, in Easy mode, with four human players. Move swiftly with full captions on and when you hear a witch crying, her music cue, or the caption, just avoid that area. Find other ways around a path with your flashlight off.
Glitch: There are several, and I'll cover them in detail.
1) Play all the way through an Easy campaign without worrying about the witches. You can startle them all you like. Dead Air is probably the best one to do it on. It doesn't matter at all if you get hit by a witch, and you can just play through without worrying about it. You have to have four human players.
Now, when you get to the finale, DO NOT open the opening saferoom door and vote to return to the lobby. Now, in 'Edit Game Settings' change it to chapter 5 (which should be the finale) and start it up again. The reason this works is because the game acknowledges the fact you've played through the whole campaign, but it says you never got incapped, never healed, and so on (so you can unlock several achievements at the same time using this). Go slowly through the finale because you have more chance of a witch spawn if you're going quickly.
I hope this helps.