Witches are a rare hostile mob, however they can be killed relatively easily. First off you want to set them on fire, this immediately causes them to drink fire resistance, and while they're doing that, they can't poison or throw anything nasty at you. During this time deal as much damage as possible, keep hitting her and don't stop until she dies. Of course if you have ranged weapons like bows then use them instead as then you'll be well out of the witches range, however you will need to be able to hit her otherwise she can take a while to kill as she heals up.
Also, in terms of witches abilities they are just as mobile as zombies (in terms of navigating your defences, so they cannot fly or climb walls).