This one is a little more tricky than you might think, especially if you do not have a group of four.
You have to go through Capter 2: Sugar Mill in the Hard Rain campaign. Even though you backtrack through all this, you only have to do this one level. Having at least one partner on this one makes it a great deal easier and just to make sure that your AI teamates dont trigger a witch, switch the difficulty to expert and kill them off, then switch back to easy. (Do this every time they come back just to make certain they dont spawn at an inoppurtune time.) Try to have at least one person with a melee weapon at all times just so you dont hit a witch randomly.
Chargers and Smokers are your worst enemies here as they can ram/pull you into a witches area. Turn your subtitles on and whenever they are "heard" stay back until you can see them and kill them. Try to only throw your pipebombs and molotovs into areas you KNOW there are no witches as a close explosion will trigger her. Now, if a witch kills one of your players that is ok, as long as you do not kill her, your are alright. So you may be able to get really lucky and just book through an adrenaline.
The biggest problem area for people trying to sneak through is the small cornfield. The easiest way is to walk on top of the pipe through the middle of the field and stop as soon as you exit the field to look around as there is usually a witch or two right outside the final building. Any questions or specifics you need help with, comment.