I don't think I quite understand the automatic demon fusion comment. Are you talking about the recommended demons when you start the Cathedral of Shadows app? Just want to make sure I'm not missing something.
Made it through that hard part, and then my incredibly unbalanced party made mincemeat of the next two bosses. It really is a game about preparation, and I can see why they give you a strategy guide when you buy it. It feels a lot like Demon's/Dark Souls in that you have to know what you're walking into in order to survive. Thankfully, the ability to save anywhere makes it fairly painless to try again when you get slaughtered by a boss. More games, especially turn based RPGs, need this kind of save system!