The whole game is extremely good the Sans-like fight at the end where you just spam the dodge skill & the slide with shadow walkers aka spamming i-frames. Was boring as hell & sucks.
Don't rip off Undertale next time if it's not going to be fun. Boss fights like that are not fun. You said it at the end that this game was meant to focus on gameplay & you cockblocked us from the catharsis of weakening & killing a boss at the end. It didn't felt fulfilling nor anything.
Think about boss fights like that next time. They're for story based games not metroidvanias like this. Thats it. 9/10 game man well done & thank you for the experience & RO nostalgia.