If you've got a gun in a crowd, you don't really need to aim much to hit a lot of people. The tape part was facetious, but not that facetious - the system wouldn't need to be complex at all.
And no, aiming is not really that hard (unless you have a long range, or high precision requirements). Lots of non-genius hobbyists have made systems that identify short range moving targets, point at them and shoot (look on YouTube). It's a "science fair" level project that I could have done (with current tech, anway) in high school. If you actually had someone experienced in machine vision and robotics, you could identify specific people and shoot off their index fingers from 200 meters, but you wouldn't need any special expertise to just shoot some dudes.
And being on a quadcopter actually makes it easier, as you don't have to kludge together a bunch of motors/servos (like the people fooling around with paintball guns on YouTube): you can use the quadcopters movement (the hard parts of which are a solved problem from your perspective) to target primarily by translation. Simply put, you just move left until "that moving thing" is in the middle of your field of vision. And, within 10 or 20 feet, as long as your gun is pointing basically the same direction as your camera, you're done. You probably hit a good percentage of the time.
Now it's quite possible you wouldn't get many shots off with each drone (though I think you would, as long as you put any thought into your gun mount and cartridge management) - but if you had even a modest budget you could make up for that with numbers. Again, it's a high damage, "high terror" attack with modest budget requirements, modest expertise requirements, and low personal commitment.
(The reality is that there's lots of attacks like this that you could cobble together if you were smart - this isn't a new thing with drones. But drones do give you options that are particularly problematic to defend against).