Went back to beat the oni boss by half-accident not really trying since I didn't want to waste my potions before a "real" attempt. I used way fewer than I expected and had plenty left over, turns out that thing is indeed super easy to cheese by sliding and staying close. I also used the kanabo to give him a few whacks in-between bouts of sliding back and forth, and started the fight off with six blasts of air spray but I dunno if that did much honestly. The kanabo ultimately did him in way quicker than I expected.
e: It's the fucking kanabo. It does crazy damage. I accidentally beat him again trying to see if it was viable, using a single potion just for security since it's free, starting off with 190 HP. Just keep whacking him with it, he goes down ridiculously easily.
I then failed to notice the obvious spirit thing you get for the nekomata form and thought "oh you probably need to also beat the harpy in the same session to get it." Used up all my potions, had to get her with the spray instead of the kunai since I didn't have any which sucked ass, somehow didn't quite die and had the worst, most nerve-wracking platforming session ever afterwards once I realized my error. Game could probably do with a skip boss button for reruns.
Anyway, how does the tower thing work? I noticed you can pick a save file for it, but does it overwrite that save or does that let you start off fully upgraded or something? I don't wanna test it out only to discover it's the former.