Normally I'd suggest more armed concert-goers, but this is different; no civilian armed with a gun would have any idea what was going on due to the way in which he attacked. In this case, it really falls on the hotel security. They let a man stay in one of their suites for three days with ten firearms, including the bullet-hell rifle he used to pull this off.
There is no real legislative way to have prevented this. If he used an automatic rifle like I think, those are already illegal across the country. If he used whatever you're suggesting, he would have gotten it regardless, because of how easy it is to get illegal weapons. Just look at Chicago.
If you want a good suggestion, it'd be to remove the motive, through better mental illness treatment and just teaching better morals in general. No one should be thinking like this guy, clearly.