This is a copy-paste from a comment I posted in the review for the game; my scores are at the end:
After a little more time spent with the game, along with use of a save state right when a boss shows up, I have figured out how to beat a boss: shoot the round core at its stern a few times. I’m not sure how many, maybe 5 times, but shoot through the body of the ship all the way and make that core fly away and come back a few times, and you get 10,000 points as the ship disintegrates. It is, of course, much easier if you have collected 5 of the ‘E’ items, which grants you invincibility and a powered-up gun for a short while, and I took down a boss that way once, but it is indeed possible to defeat a boss with the standard shot.
Also of note, when you’re invincible, running into the normally impenetrable rising walls will destroy them for 3000 points each, and the impenetrable flying objects can be shot and destroyed during invincibility as well (not sure how many points, if any, though, since no number pops up for them). And, the game does appear to loop indefinitely, as I finally started recognizing certain landscape layouts repeating. Still no idea what it means when the landscape suddenly starts tinting and the lines on the ground start glowing.
Playing at Normal difficulty with 3 starting lives, my high score from game start: 212,650. It’s worth noting that bonus lives are earned every now and then, but I didn’t pay attention to what milestones did that.
Before my most recent attempt, which achieved the above score, my high score from game start was 65,660, and at the time, my high score from way into the game, reloading a save state that was right at a boss and playing as far as possible from that, was 85,220. Somehow, I smashed both scores with that most recent game.