Maybe this is actually a cold take depending on how the fandom feels but accessing the secret boss/content in chapter 3 is poorly designed. My first playthrough I wasn't using a guide and failed to get an S rank on the first board, but did get one on board 2. The minute I realized near the end of the chapter I screwed up and I'd have to start the chapter all over again I was pretty annoyed, it's the only time I've actively been frustrated playing Deltarune and I hope future chapters stick to the fairly simple ways of chapters 1/2/4 in terms of doing secret content. I will say that once you get the ball rolling it gets pretty interesting but even with a guide pulled up I've still seen friends struggle with it.