Hmm...
First off, decent, original game.
Second, it was preeetty easy once I figured out you can hit anything odd with a 3, then just spam 2's until you kill everything. Maybe a version with less "enemies" on screen, but you've got to kill them with 3, 5, and 7? Or something like bubble-bobble, with different numbers getting shot every time... or factors are good too. Dunno, something with a bit more thinking to it...