I got a killer strategy!
All I do is go into a corner and kep shooting nonstop. When an enemy flies towards me to attack, they are caught in my line of fire and are so busy thrashing around because of frequenty damage that they can't get a shot in. This sounds foolproof, but those pesky buggers always use kamikaze runs to kill me. The farthest I've made it is wave 5. Hmmmm....maybe I should get better strategy.