Shoot the enemies and don't let them become 10. Careful with bullets that don't hit.
The ability to pause makes it too easy, while the spawn rate without pausing makes it pretty difficult. It also doesn't seem to count missing bullets as enemies. I got to score 150 with terrible 0... and just got bored. There isn't a way to "win" either, is there? And I didn't figure out what do the yellow enemies do.