IF Paddock was the sole shooter, which is not certain at this time (we don't even know if he fired a shot. I know of no reports of muzzle flash from his alleged shooting platforms), then a Barrett .50 would have ended his spree quickly.
A competent marksman with various range capable arms could have kept his head down, at worst, and ended him with a good shot. Most bolt action .30 cal rifles are 1/4 MOA capable.
Most shooters aren't.
TBQH, any gun in the hands of someone ready, willing, and able to take it to the murderer would have solved the problem. A Raven .25 (if you could keep it from jamming) and a 32 story elevator ride would have worked.
Claims that nothing could have worked are put forward by those that don't know any better.
I have personally disarmed 3 persons in separate incidents who were threatening me with firearms. No shots were fired, and no police were involved, then or later. Those people never threatened me again, and hopefully learned to practice better gun control.
If someone has a range advantage, which is all that firearms give you, the solution is to close range.
It's counterintuitive, but effective.
I think I have an explanation for the lack of muzzle flash from his hotel suite. He was firing blindly through the curtains and blinds using his bullet drop and known angle perhaps. Also, there's this:
Keep in mind that there's missing spent brass from the hotel room as well. There's not enough spent brass there to explain all the shooting. A second shooter from a different distance as the video explains would resolve that issue. It also helps resolve the lack of muzzle flash from the hotel suite.
Either way though, we are not being told the full truth of the situation. Heck, just go listen to the official police scanner transcript: