Kenobi lacks the offensive ability to press Zannah hard in a duel. If she can defend successfully against Bane then Kenobi's rather flaccid in comparison. Zannah is faster than him imo and thats enough for me to believe she'd come out on top in a lightsaber duel.
Also her cousin was like, 13 or something. And the Jedi were aware of her I believe, but obviously not expecting it. She does levitate one of them while killing him in the comic though.