The problem could be easily solved if the US invested in a solid mental healthcare system. Most of these multiple shootings are committed by people who suffer from mental illness, and there is no safety net to catch them. Care for such people is limited even if they have health insurance. The clinics that provide services on a sliding scale are generally just cattle calls. Patients get put on medications, but there is very little follow-up to be certain the meds are being taken and the patients are functional.
Americans will never willingly give up their guns. Since that is the case, the US government has to do something about the mental health system in order for these tragedies to at least decrease. Also background checks should be done for anyone buying weapons, and that includes relatives who are mentally ill and live in the gun owner's household. Had that law been in place and followed by Nancy Lanza, Sandy Hook would've never happened. Her son, Adam, was clearly troubled, and there is no reason in the world that she couldn't have stored those weapons in a secured environment outside of her home.
Nothing will probably eliminate mass shootings, but the US has to put some programs in place, so they're not so frequent. That is, of course, if we don't want to live in armed camps. Schools are already arming personnel. I wouldn't be surprised if malls, stadiums, and other public gathering places aren't next. I live in a small town between Cleveland and Youngstown, and there are armed guards at grocery stores in both cities.