Wish there were some kind of scoreboard at the end after you died that showed what level you got to; I made it to somewhere between 25-30 and was waiting to take a screenshot until afterwards.

I did well by only ever firing upward +/- 45 degrees (except the occasional horizontal shot for enemies stuck on the bottom), and then staying near the bottom of the screen and moving myself into position before accurately firing (rather than spraying and praying everywhere).