Yeah you did well to avoid story spoilers but you did get spoiled on game's premise, which is arguably more important, so now you pretty much are playing it wrong. Undertale is best when it's subverting your expectations but it falls flat if you don't have those expectations in the first place. But to be fair, this is a kickstarter indie game from a creator previously best known for making music and who had only developed an earthbound ROM hack up until then. By all means, undertale should've been a niche game and it's weird that it blew up as much as it did. I don't fault the developer for designing his game with the assumption that people will treat it the same as the billion other indie RPGs.

Like with that boss you mentioned, she's designed to be challenging for a player with good gear and a high LV. Then you beat the game and are told to replay it without fighting. Then during a second playthrough, you remember this boss when you reach the point where you can buy her key item. Then you're relieved that you don't have to fight her again, but without your gear and stats.

I feel confident in saying that you're not meant to know in advance that this boss is optional because you're not supposed to know at first that fighting as a whole is optional. The key item is better understood as a mercy for repeat playthroughs than a secret requirement.

This video is tangential, but I think you'd be interested. It covers what RPGs is like from someone with nearly no gaming experience at all. The player didn't resonate with undertale either because she didn't recognize the tropes that it subverts so it just came off as boring and irritating.

https://youtu.be/YBRTg74ZhLU