If you’re looking for gnarly shit, I highly suggest playing the weird route. When you start a new chapter 2 save file, backtrack ::: spoiler spoiler when noelle first joins your party. Then try using her ice magic. :::

The deltarune wiki has an article detaling exactly how to get it (you will get mildly spoiled): https://deltarune.wiki/w/Weird_Route/Chapter_2

Chapter 3 doesn’t really have content exclusive to this route, and in chapter 4 it will be obvious what you beed to do.

I agree about Tenna. Most of the fandom’s favourite characters are secret bosses or incredibly obscure and mysterious. If you don’t do a 100% playthrough you don’t get the full story of deltarune, it’s sad.