Unfortunately the "Witch" is a non-playable character. The boomer, hunter and smoker can be 'pushed' back with your weapon bash attack, however this does not effect the tank or the witch. When the Witch attacks, she will instantly head towards whoever disturbed her. She runs at you with incredible speed hitting you incessantly doing a lot of damage while knocking you back, preventing you firing upon her. You will quickly die against her unless you can keep her a good distance away.