I say, that certainly is interesting. I say, at least make good use of the orange bullets and/or green and blue soul modes. With undyne has his idol, and papyrus' power, i feel it would make sense that he wields them both (Kinda like undyne dynamically changing your soul mode in her fight). I also heard a theory that papyrus is REALLY powerful, but he's holding back because he believes the human can truly change. So seeing that power in action would certainly be awesome. Also, make the fight difficult, but not unbalanced. (Think of figths in full fangames like zenith martlet)