Is the ambusher a trained fighter? If so, then your MC is not going to get a lucky strike. If the ambusher has skill, she isn't going to get lucky. There's only so many times the old airplane-blade-chops-someone-to-bits trick can work. We've got to be able to buy that she won the fight against an experienced fighter. Otherwise it's just a cop out.
Make sure it's believable, and for it to believable, she's got to be facing someone with little to no skill. Not sure how that will work in with your story.
I disagree. A few years back I did taekwondo for awhile. In one of the first classes me, the lonely little white belt, was put in the middle of a circle with a black belt and told to close my eyes, then the black belt would grab me and I'd have to figure out how to get out of the hold. The person in question was a friend of mine who thought it would be funny to put me into a hold I wouldn't possibly be able to get out of (behind my back, arms around my arms and neck, I don't know what it's called but there's a name for it). Now, she was supposed to just grab my wrist, for example. I knew she'd do something crazy like that, though, and I remembered having heard once that if someone gets you like that you can drop out of the hold. I'd never done it before, it was just something I'd heard. So I, completely inexperienced and blind, managed to not only get out of the hold, but to pin the black belt on the ground.
Yeah, I wasn't some random person who knew absolutely nothing. I'd taken a couple of classes at this point and heard advice about self-defense even if I'd never had to use any of it. The point is, I essentially got a lucky shot, and I think 90% of the reason I did is because the other person didn't expect me to.
It's entirely possible that a newbie might be able to take down someone more experienced in the right circumstances, especially if the more experienced person is overconfident and not expecting a fight. The point is to surprise someone, and I definitely believe it's possible.