Left 4 Dead 2 - Versus
Nothing much changes for the survivors for a witch in versus mode. They tend to be more rare in versus, although you'll probably run into one every few games. The only trick is being a little smarter. The witch is not a playable infected character for versus mode. If the infected team has at least one brain amongst them, then they will try to get you to set the witch off.
There are a few favorite tactics. Naturally I suggest all of the following as good strategies for when you play as the infected.
Jockeys can try to steer a survivor into a witch, although your teammates would practically have to be asleep to let this happen.
The smoker is much more of a danger. He can stand behind the witch and effectively use her as a shield. Survivors will need to be pretty quick with some very well aimed shots to keep the smoker from pulling someone into the witch. If someone does get smoked, shoot the the tongue or bash the survivor. Firing at the smoker will probably just make sure that the witch comes for you.
The boomer is also quite useful for witches at choke points. If the survivors set off a boomer next to a witch, it will still startle her. If she's fairly close to the main path, it will also let you stand around in plain sight and taunt them. You can even vomit on them without much of a chance of retribution.
Any other special infected character can also use the witch as a shield. You'd be shocked at how many survivors will try to snipe a crouched spitter with a shotgun. Even experienced players get careless and take risky shots without thinking about the stray rounds.
Once the witch has been triggered the infected should try to stir up some chaos. That will keep them from covering the downed survivor. If played well, with a good boom or spit, it should be enough to kill a whole team. If you want more information on good versus tactics, then check out our guide for playing the infected in versus.