Well, when I play with my regular team mates together, we mostly avoid killing the witch.
But if you absolutely need to, there are two ways:
Fire and kite: Unless the surroundings/terrain is flat wide area with no place to hide or climb, let one with a mid/long-range weapon shoot the witch and start running around IMMEDIATELY. While she's chasing the guy, the rest of the team unloads on her with whatever they have. Depending on difficulty she drops dead in less than 10 seconds.
Don't try this on Expert, especially if your team is not good at tracking targets while firing.
Crowning: If on campaign, the one player with the auto-shotgun gets near and gets the fast kill.
On expert, it is possible to use the shotgun, but is also very risky. If the first shot misses a single pellet, you're dead. So, if you have no other choice and only the shotgun at your disposal:
Get real close and aim for the chest, AS SHE'S STANDING UP. I do it like this: I get close to touch her, crouch and keep my aim on the head. As she starts standing up, I pull the trigger the moment my cross aim is at the middle of her chest.
On versus, a good team usually has already set up a trap around her, so be extra cautious. Don't even try it with the shotgun. If you miss the first one and there is a boomer around, you're going down pretty fast.
If you are soloing, and NEED to get that witch off the street, you can also use the sniper rifle from a safe distance.
The first shot HAS to be headshot, which causes her to stumble and give you about two extra seconds to unload further.
UPDATE: As of December 3rd, 2010 Update, the witch can't be killed by crowning (1 shot in the head with the shotgun) in Realism Versus. All other methods still work, though.