I am already using Lott's data.
First thins first. Do not use Lott. He is completely unreliable. If the person you're conversing with bothers to do the research, your credibility will suffer.
Since this is his article you have to find errors in his argument. Don't present your own argument. That keeps you on the offensive.
Probably the best theme to use in a statistical argument is to insist that your opponent prove causation. The saying goes "correlation is not causation".
eg states that went for McCain over Obama have a higher correlation with violent death. That doesn't mean that voting for McCain causes violent death.
http://www.theatlantic.com/national/archive/11/01/the-geography-of-gun-deaths-/69354/
The odds are that your opponent is simply parroting some other writer. If he has links, look for the original author. Frequently a web poster tend to edit the original source to bolster his argument. You might be able to find that with this guy.
If he is using an academic source, that source will have qualifiers instead of outright assertions regarding that data. Use those qualifiers against your opponent.
Finally, it is a mistake to think you have to make the anti- gun control argument. Just as there is correlation between gun laws and gun violence, there is even stronger correlation between violence and poverty, education, race, and a host of other statistics. You could instead propose alternative solutions to gun violence that would be more effective than your opponent.
For example, Coral Springs, Fla. and Orlando, Fla. have similar gun control laws, yet Coral Springs has a lower incidence of violent crime. Instead of indulging in regulations that have doubtful efficacy, perhaps we should find those positive characteristics of cities such as Coral Springs, Fla. and apply them to cities such as Orlando, Fla.
http://www.infoplease.com/ipa/A0921299.html
Then make the argument for your exemplar cities and those more effective efforts that you choose. It boils down to "you might put out a house fire with a garden hose, but why not use the fire dept. instead.
BTW, the District of Columbia has one of the highest incidence of violent death and went overwhelmingly for Obama.