Unfortunately, this does require you to have saved before it, because there's no way to access the menu after. I fast-forwarded back to that point from an older save.

The save files are at "~/AppData/LocalLow/DefaultCompany/Project Eden's Garden". I think on Windows you can open that folder that using Win+R then typing "%userprofile%\AppData\LocalLow\DefaultCompany\Project Eden's Garden". For those of us on Linux, you have to find the Wine prefix for the game, Google can help you there.

Open your save before the bullet hell section in Notepad. It's a JSON file. Near the beginning of the file you'll find "action":2. Replace the 2 with a 0 for easy and a 1 for mid. Then just save, and load the save file in the game. The bullet hell section becomes pathetically easy on 0, which is what I did.

I would suggest to the devs that perhaps you could be prompted to drop the difficulty if you fail that segment like, 2 or 3 times.