In my experience, anti's dismiss such stories as aberrations that don't justify the murders, suicides, and hurt children. The best approach, one I have successfully used to change and anti's stance, is to get the CDC study and the before and after violent crime rates of places that put in place a draconian gun policy.
A good place to start would be:
http://www.justfacts.com/guncontrol.asp
Another good tack is to be sympathetic to the apparent goal. After all, we really do want to see a drop in gang violence, murder, and suicides. We need to look at the root causes and address them rather than the tools used. Violence and accidents existed before firearms and will continue after every gun has been melted down. Also, education and awareness are important. The world isn't safe and never will be. We can educate people on situational awareness, personal security, and the benefits of strong communities.
I would refrain from introducing the idea of everyone should get a gun, that will close them down. If they bring it up, I'd recommend the response "I'm not here to debate firearms, I'm here to help find solutions to the problem. After all, gun bans aren't a solution, as seen around the world. Banning guns doesn't work, lets look at other ideas."
Finally, if you've been completely dismissed, stonewalled or they've taken to the stance that gun bans are the only thing "you're pushing a political platform, not looking for solutions"
That's my two cents, best of luck to all that go. Sorry I won't be able to make it.