If someone is firing on you first, and your in the open, then they must have gotten the drop on you to some extent.
They are likely stationary, likely giving them a good opportunity to draw a bead on an exposed target (you).
You best chance in this case seems obvious to me. If you're not hit with the first shot, get your self to some cover or concealment. He has all the advantages. My first objective is to not get myself shot. You'll have to pull your weapon, aquire a target, try to place a shot, while the BG already has you in his sights.