Yes, I agree with you on the difficulty. We really ran with the thought "how would a bullet hell work with only one bullet?" that lead to "how can one bullet be as terrifying as thousands of bullets?" And the answer we found is: make it hellish.
We did beat the game once before uploading it, so we knew it was possible (Mario Maker approves this line of thought?) But I believe some balancing is in order. The time limit did not help with this fine tuning.
And I'm so glad you liked our game!
Thanks a ton for this thoughtful feedback!