Could be tricky. Unless you can get close, most flashes won't have sufficient power to provide enough light from any appreciable distance. It may be that a flash going off would also spook them, so that you would only get one attempt.
Is it possible you can try and shoot them when there is still some light in the sky? Not necessarily broad daylight, but enough to be able to shoot using ambient light - probably using a tripod to combat the resulting slow shutter speed.
The only alternative I can think of is to use the sort of lamp that rabbit shooters use, to provide light to illuminate the deer.