Here's my best run so far. I killed the level 3 boss' tank thrice, for disclosure's sake.
There are a few spots with high point value enemies (choppers, for instance) where it's a matter of going back and forth to spawn a few extra; these are not infinite though. I watched the video mbd posted and that person missed a few of these by carrying on sooner than I did. I also got some decent extra points by going across all 3 bridges on the second-to-last level.
For beating the last boss without dying at all, I found a safer way. If you sit at the bottom left or right of the screen (I prefer left) he will almost never shoot the big cannon at you. Hug the wall, and pop up to shoot up and/or diagonally at him when he approaches. He killed me only once while I did this on one of my (many) runs.
Great game, I enjoyed it a lot.
Score: 525,040 (517,040 + 8,000)