As I see it the biggest obstacle to accurate return fire initially would be the panicked crowd. Once the crowd is hunkered down, then the only problem is the absence of light and the smoke. Night sights would be a plus if you remember to use them. A semi auto of sufficient power and capacity fire bursts of 3 or 4 and shoot him to the ground. I would say point shooting training would be a big plus. It certainly is no place for a 5 shot snubby and no reload. Like has been said it may take 4 HITS to realize he is not going down.
It still hurts to get hit with a vest on, the military kevlar helmets will not stop a 223, 7.62X39 or a 40 S&W that I know of. In the helmets I shot all went in the front and out the back. I was really surprised, those are the only rounds I used as they were pretty shot up.
My rifle and pistol are tools, I am the weapon.