I'm months late, but thanks for your comment. I would have loved to have more opponents with multiple growth stages, but I couldn't find a lot of sequences on the net and my photoshop skills are very limited.
The Bonus fight is difficult, yes, but you have all your potions (except the revive one) for one fight instead of having to divide them between 6 fights.
As for endgame, well, I guess you already went on some dates with the shopkeeper, but did you know you can actually fight her? There are two ways to do that. The first way is to do a Bonus fight while having the gallery fully unlocked, and the other way... is learned by playing videogames with her after having done all the dates!
Also, you can try to transform all the teachers from the tutorial. It's kind of a mini puzzle.