Move to cover, if you don't know what cover is keep moving until you find something that is. It's hard enough to tag a specific target from 500yds let alone a moving target...and this guy wasn't picking his targets very carefully, certainly at first. So, as John Belushi said: Serpentine!
Me? I stay out of crowds...I don't even like mildly crowded stores, the spidey senses start going crazy. And I hate Vegas, been there too many times for work, if I never go there again it will be too soon.
In this specific situation; unless you had the room next door (and carried a similar arsenal on your vacation) there's not much you could do offensively so we're back to protect your loved ones, move to cover (and don't go there in the first place).
Just a thought....It truly sucks that we live in a free country and by the acts of a few the many are seriously restricted.