This is such an impressive demonstration of artistic flexibility. I think my only real gripe is that Undertale-Bowser looks a bit *too* clean and it feels like it has a smidge too much detail. Most of the battle screen's sprites in that game are sort of known for being a bit rough around the edges. That's not to say that it looks bad, though; I could see it being more of what you'd see if there was an official collaboration between Nintendo and Toby Fox that involved Bowser in Undertale, for whatever reason.
Again, though, that's just one sprite of fifty! If I were grading this, the worst I could give it is 98%, which is still an A! Outstanding work, as always.