The later levels have been modified to be way too difficult for how punishing this game is. The complete spam of spread shot big bullets is just too much and completely covers the screen. If you want a game this difficult there should be checkpoints at the bosses rather than a full restart on failure. I do really enjoy the game, but this is simply over tuned. If you want to make the later levels a threat for late game set ups that can wipe each wave in seconds, it might be better to have higher HP on later enemies so that they don't die as easily.
Thank you! Here I was thinking we made the game too easy...
We are currently working on an update that will add more roguelike elements and hopefully make dying less frustrating. e.g. Permanent upgrade shop to make runs easier over time and more control over how difficult the game is. More builds to allow for different playstyles. More enemies and attack patterns. We'll keep your feedback in mind when we are rebalancing with the new content.
This is also the first bounty of many planned, so you're right that it shouldn't be too difficult.
Glad to hear you plan on continuing development, the concept is simple but pixel art style is always great and with a little spice mixed in there as a prize it gives some real motivation to win lol. I haven't played very many straight bullet hell games but I have played plenty that are akin to them so I hope my input can be of use.