You need 4 hits to kill her, and you will need to dodge one of her attacks.
Watch this video on how to kill a witch using melee weapons.
Best place to find witches is in Hard Rain campaign, chapter 2. There are usually plenty of wandering witches.
If you have friends, easiest way to farm witch is to play the "Four Swordsmen" mutation with 3 other friends. Bots tend to mess up and will "help" you kill the witch. If you are playing solo, I recommend starting the game with local server, and use the "kick" command to kick all your AI teammates before trying to kill the witch.