I got as far as this boss, beat it, saw the Earth explode, decided I had played enough.
He was a very satisfying boss, but I recall absolutely hating this one that was an eye in a narrow tunnel, and this sunflower-looking guy.

The game is a lot of fun when you've got a good flow going, but when you're up against a boss and your weapons either don't work or are entirely out of ammo, it's miserable.