This sounds a lot harder than it is. I did this entirely by accident very quickly when I started playing and I'm fairly certain I did it a number of additional times as well. Since it wasn't intentional I can only speculate about a precise solution.
Start a new game and choose defend the town. You'll get a series of "quick draw" matches against one or more opponents. Don't try to be super quick and kill them immediately. You want them to be shooting at you. I don't think they can actually hit you so speed doesn't matter except for your own joy. Beyond making sure they're shooting at you, I'd try to aim for their gun to increase the probability you'll unlock this (the physics seem very generous). I believe when you shoot a bullet there is this huge clang sound (like a bullet hitting a frying pan). The achievement should unlock immediately.