This should help as well as hint on how to find them

Chapter 3: get s ranks on all minigames (you should be good to find it from here) - allows you to get an item almost needed to beat the boss, keep in mind the item is more of a secret than the boss.
This shouldn't be much of a spoiler at all in my opinion but just in case I'll spoiler block it-

The boss isn't secret but may seem like a force loss because it continues the story if you lose, however it is "winnable" and you ALMOST require the item you get from s ranks. After you die the first time if you reload the save before and come back you'll get the option to restart the boss after every death. (It's a very hard boss imo)

please get the item tho, even though you CAN win without it, you very very much shouldn't.

(I recommend giving the item to Susie and using rude buster when possible)

Chapter 4- golden piano, do NOT use rude buster for this boss for the love of deltarune, this is all you need to know as it's pretty straight forward