To the OP: Great topic! I know I struggle with this myself sometimes. All of the above are great suggestions. I find analogies and examples of other, non-gun situations to be more helpful since most people who are anti-gun are totally ignorant with respect to firearms and cannot relate to them. If you start spouting off about technical firearms jargon, you'll lose them and they'll label you an "extremist gun nut'.
There's always the argument that it's a Constitutional right. For example, one could argue that swear words and profanity have no useful purpose and that no good comes of their use. So why shouldn't we make it a crime to use those words? After all, we could still use all the other words, and profane words were only coined to do damage, hurt feelings, and offend people.
So why not ban profanity? Well, because free speech protects all speech--speech that does good and speech that does harm. Benjamin Franklin said that those who sacrifice liberty for security deserve neither. I couldn't agree more. Do you make right-infringing rules for the 0.1% of society who commit crimes, or do you make them for the 99.9% of people who obey the law?
I always like the car analogy, too. Should we ban sports cars? No one can drive 140 mph anywhere in the U.S., anyway, so why should they even be allowed to be bought? They're just designed to be driven over the speed limit, which is dangerous. So if you go out and buy a Ferrari, then you're going to be out driving 140 mph on the expressway before you know it, right? The fact that you can means you will, right?
Wrong. You control how fast you drive and what you use your car for (drunk driving, etc.). If you want a Ferrari and can afford one, then why shouldn't you be able to have one? The semi-auto being used to kill as many people as possible analogy is like saying that Ferraris were designed for drunk driving. It's taking a HUGE leap between two things in society--guns and violence. Yet we don't draw an association between car subtypes and drunk driving. Red sports cars are no more able to kill someone than a Chevette or SmartCar. Do you think the pedestrian who gets run over by a Honda Civic going 50 mph cares that it wasn't a Corvette going 50 mph?
By that token, if someone is shot with a .22 LR bullet out of a bolt action rifle or a .22 LR bullet out of a Ruger 10/22, do you think they know the difference or care? What about the .50 BMG out of a semi-auto Barrett or it's bolt-action counterpart? Does it make a difference? Is the rate of fire what kills someone, or is it the bullet flying out of the muzzle that's aimed at them? Once someone has made the decision to kill someone (making them a seriously disturbed individual), they will find a way to do it one way or another--semi-auto, knife, baseball bat, or otherwise.
Then there's the argument that criminals disobey the law, so how will new laws stop them from doing so? It will clearly only disarm the law-abiding citizens (i.e. the victims). If criminals disobey the laws against violence and murder, why would they obey a firearm regulation? Moreover, even if all guns were banned tomorrow, criminals would still have them.
Look at drugs in the schools. Show me a high school that doesn't have drugs in it or at least a few kids who attend that school using them. How could they have gotten them? It's not like alcohol where they can just pay someone older to buy for them. Drugs are illegal everywhere and to everyone! So how could they possibly have permeated our society so easily?
It's the same reason Prohibition was unsuccessful at ridding the nation of alcohol. You cannot legislate morality, and banning inanimate objects will never substitute for personal responsibility. Banning and limiting things merely makes them profitable to criminals via laws of supply and demand.
As far as attack from multiple people being unlikely, so is needing life insurance or having your house burn down, yet you still keep a fire extinguisher and get a life insurance policy. Plus, why should you not be able to optimize your odds with a semi-auto in a life threatening situation? Is your life worth jeopardizing b/c of what Mr. Cho did at Virginia Tech? Are the two related at all? I don't think so. Who is the government to tell us how we can and can't defend our loved ones and ourselves when our lives are in danger? What if you were only allowed to have a life insurance policy for $10,000? Maybe you have a lot of debt, and that won't make a dent in it for your family. But the government tells you you're not likely to die, anyway, so it's limiting your policy to $10,000. Or maybe you're only allowed one smoke detector or fire extinguisher in your home. What if you have a 5000 sq ft home?
The list goes on and on. Stay calm in your discussion, as hard as that may be. I get so irritated by the paucity of logic in the anti-gun arguments sometimes that it can be hard to not get a little annoyed or fired up. Good luck in converting her or making her a little more well informed. Lastly--take her shooting! That may also help dispel some myths.