As the heartless cynic would think: Shoot the other guy in the knee. You only have to be faster than one person. The caliber is irrelevant.
If a bear is chasing two people and one person falls down (for whatever reason), would the bear always go after the downed person? It occurs to me that I can't think of a reason that the bear wouldn't continue chasing the moving, and thus more visible target. But then again I don't know bear behavior at all. Can the more informed help me out?