It is very hard the first playthrough, or at least boss battles are. Boss battles are outrageously unfair at times. It takes a lot of careful planning because you will not win a Boss battle with brute force. You need to know what kinds of attacks they use and learn how to prevent them from getting extra turns.
After your initial playthrough the game is a cakewalk though. The anti-grinding features are removed, you get to keep all your powerful endgame demons in NG+, and you get to keep the powerful endgame skill you unlock.
If you really are struggling there is a way to get rid of the anti-grinding features early. It will still be a challenge, just much less so from what I hear.
What you would need to do is
It is considered the easiest route in the game. Once you complete a single ending you should have no trouble on subsequent playthroughs when getting the other endings.
Exactly. The game is fully voiced, and yet the majority (or at least 40% IIRC) of the voice clips are for the 8th day. 8th day should be massive.