the fight with muffin is incredibly easy, you just have to make sure you have a glass of water from that horny watercooler thing and you can use Chara's promise more than once, up to 4x for a 90% damage reduction which makes all of muffins attacks do 1 damage and her servant bunny shadow thing doesn't even deal damage at that point, however since i beat it ill provide a save file through mediafire that should be good for up to 14 days, i think this is the save file? its a .ini file so it should be i think:
https://www.mediafire.com/file/47z9cikk2jc2lqt/global_save_0.ini/file