Indeed, balancing difficulty is something very hard to do in game dev, I think that stats solution is good. Personally I prefer it than a set difficulty level because some bosses are easier than others (at least for me), so it adds that extra level of control.
About length, I do agree with you in the fact that bosses should have a longer health bar but sometimes it gets repetitive. I guess that is why some bosses have a second form (or at least a different set of skills after you hit them enough). But I have no idea on how you could even implement it in an rpg or if it would fit the story so this isn't good advice.
Ohh a vn, nice. Good to see you're getting the hang of it.
Good saturday too! Saturday? Oh wait, this was posted three days ago...