Crowning actually doesn't work as well on L4D2 as it did on the first game. The witches seem to have a faster reaction time.

You can take a chainsaw to the back of her head. This is the easiest approach--get close to her, moving slowly, lights off. When she starts to turn, take the chainsaw to her head.

You can also use an auto-shot gun the same way, though it doesn't work nearly as well.

...or, if you're far enough away, just chuck a molotov at her and have your teammates shoot her before she gets to you.