They'll come up with something. Her death will be saved for something closer to a series finale rather than a season finale.
They had Kanan & Ezra duel his ass and neither of them even got all that injured. "But Vader wanted them to escape!" True, but he didnt' HAVE to want them to escape, right? Someone in that room will write up a reason for why he doesn't WANT to kill Ahsoka on sight, and so he won't.
Unless they've heard that Season Two really is all they're going to get (I still think it's possible they'll chop up Season Two into two halves, and name the back half Season Three, and call it a day) in which case yeah - she'll probably die in that duel.