I though it was hard for my first few runs, but then I got upgrades early (in my first runs I didn't get any) and just snowballed through everything until the final boss. The first two bosses took damage so quickly they reached the next phase and had their bullets despawn before they even reached me, I barely had to do any dodging and most other levels weren't much harder.
Then I died on the end boss and didn't want to start again.