Zombies killing a biled witch

Sometimes it's just unfortunate that a witch will be sitting in the doorway the players have to go through.
Sometimes she will be sitting in a place that is inconvenient, like at an event where you have to burn cans, so triggering the event will set her off.
I've even encountered her in the doorway of the safe house you are going to. (No Mercy)
If you have a bile jar, throw it at the witch and instantly go "idle." Zombies will overrun her and kill her.
If you bile her DO NOT shoot her or she will come for you. Just stand back and enjoy the show.

Do exactly the same with a molly if you don't have bile and vice versa.

Also, if you have no bile or molly, you can shoot her in the head which will stun her for several seconds while you run to the safe house. (This is not recommended if you are far away from a safe house.) The witch can run faster than you and she will catch you.
Remember, a witch cannot enter a safe house; even with the door open, so if you get there just wait about 20 seconds and she will cease her chase and run away.
If you are too far from a safe house and you accidentally shoot or trigger a witch, run for the nearest ladder and climb half way up. DO NOT climb to the top of the ladder or she will climb it and get you.
You can also use "explosive ammo" on easy and normal difficulty to kill her.
DO NOT try this on expert unless all the players are involved in shooting her.

You can also use the sniper rifle to repeatedly shoot her in the head. She will eventually die. Don't miss your shots however or she will recover and come for you.

Also she can run through water at full speed so if you startle her you're dead.
The Devs have an excellent guide on witch running speeds, with different speeds for day/night/rain/water etc so you should check that out.

Have fun. :)