I went ahead and did as Sparen suggested and installed in in a fresh copy of Danmakufu; I still have no idea what the problem was (I was unable to reproduce it on my machine), but here's said copy of Danmakufu with the script already installed. This version works on my machine. Hopefully it will work on others.
This works for me now.
This is the log file which my machine saved:
https://pastebin.com/4PTBMJ6AIf something is causing a crash, I would look at the loading of the boss scene objects, specifically Intro.dnh, because that is where I can hear my computer struggling the most to load. This happens on line 124 and 125 of the log file.
My machine also couldn't load some textures starting at line 147. I think this happens when the intro dialogue is running.
During the script, I also had some texture reading failures for taken photographs:
data/photo_41.png - line 305
data/photo_51.png - line 328
data/photo_54.png - line 336
I don't think these have the capacity to cause the script to crash, but it might be worth looking into.
On top of this, the bullet spawning of some of the patterns on Easy Difficulty is bugging out.
I had problems where bullets did not appear to be spawning correctly/at all here:
1st Summer Nonspell - 2nd phase
1st Winter Nonspell - 2nd phase
1st Spring Spell Card - 3rd phase
2nd Summer Nonspell - 2nd phase
2nd Spring Nonspell - 2nd phase <-- Maybe? I'm not sure if the pattern is working as intended or not.
The script eventually crashed after the second-last spell. There is no information regarding the crash given by the log file.
I hope this helps you fix it, because your script is really really cool.