Well, I finally managed to beat it (I blame the Condemned Area boss for keeping me stuck for two weeks).
I initially wasn't expecting that for the final boss, but it was quite impressive after a few rounds with it. Though, I personally don't think Toccata into Blood-Soaked Darkness "works" as well for it. That's just me though.
I do have a few bugs to report, too. During Epilogue Stage B, in the tall room with the wall-spears(that shoot out and can be walked on, sorry I don't have a screenshot), Efrain collided with one during a jump and instantly died. I don't know if they're instakill or not, but that isn't the problem. Efrain became invisible and the camera jumped downward. My guess is that he fell because he was airborne and became stuck in one of the other spears, but that's only a guess.
I also found that if Efrain collects two giant hearts (the ones worth twenty) at the same time, he will only gain twenty of them, rather than forty. I noticed this on the final area just before Katherina's first form. I don't know if this applies to all heart forms (since it's pretty rare for two candles to spawn the same heart every time), or if it's just for that one area; but it can and sometimes did affect whether I was able to beat her (in regards to item crashes).
But at the end of the day, though, this is amazing. Outstanding work, Mig, really.