Will you have a helper? I suggest you bring a collapsible reflector/diffuser such as this one.
Raya 5-in-1 Collapsible Reflector Disc (42") CR-51-42 B&H
The diffuser panel is perfect to hold up to block direct light on the face that is coming through the leaves. If you can't get one in time, maybe try to improvise with a thin white pillowcase.
For the lens, I recommend you use the 50/2 for the full body or torso shot, and the 135/2.5 for the face. Stop down slightly (f/2.8-4). The 50-200 will not give nearly as good a result.
Regarding the flash, it is OK to use it for fill if you must, but keep it dialed down 1 stop. You don't want its light to be obvious in the photo.
One last thing. It will save you tons of time and frustration if you white balance your camera. Have your model hold a gray card (or clean white paper) by her face, and use the custom white balance feature. Do it every time you move, as the light will be changing depending on the time of day and the amount of leaves it goes through.