Here are some points for you to consider.
1). A pistol will win against a rifle IF the pistolero kills the rifleman before the rifleman sees him. Much of this has to do with tactics, but that should be part of your skill set.
2). If the pistolero fails to drop the rifleman due to an inoperative pistol, a miss, or insufficient damage caused by too few pistol rounds, the rifleman will probably kill the pistolero.
3). The surest way to stop a man in his tracks (with some permanence) is with a shot to the head delivered at close range. Some folks dislike teaching head shots. I require them.
4). Verbalizing or challenging, as in "Halt - Drop The Gun" is not a good idea. In fact, its virtual suicide in a situation like this. You cannot yell and shoot well at the same time, and yelling may alert the bad guy to your location. There is evidence that this occurred in Tacoma.
5). You verbalize and challenge ONLY when you are not certain of what is going on and then ONLY from behind solid cover (if the adversary is armed with a rifle, few things qualify as cover). If you have enough to justify shooting, then shut your trap and work the trigger.
6). There is a great deal of discussions on Defensive Shooting. Shooting in a case of this magnitude is only defensive in concept. It is pro-active and aggressive deliberate and premeditated shooting.
7). Although I like and teach point shooting (in its context), pro-active shooting like this requires traditional marksmanship. Be deliberate! Watch your front sight and press carefully and repeatedly (with your mouth shut).
. Train with photographic targets and do force on force training. Get used to the idea that you carry your gun to potentially shoot at a human attacker. The more you get comfortable with this the easier it will be. Tommy Lee Jones' character in the movie, The Hunted, told his students that when you can kill in your mind, the physical part is easy. He's right.
9). Consider that under some circumstances you may have to make do with a knife. I'm not saying that you will attack a rifle man with your Spyderco at 50 yards, but that in the event that he is within arm's reach (which was the case with several of the Tacoma hostages) your knife may be the only thing between you and being killed. So learn how to kill with your knife.
10). Last, consider that there may be a reason why God (or fate if you wish) put you, armed and trained, at that very spot and place in time, with the ability to save innocents. This is the life-giving sword (or gun in this context) that some ancient warriors discussed in their writings.