Whoever told you that Nocturne is unfair is not your friend and a liar. It is challenging, which you already realized, but I don't think it's ever unfair.
Did you fight Matador? That's the first Skeleton, and the boss that really tripped people up. One of the most challenging bosses in the game. If you beat him, you can beat the game. The other Skeletons are all way weaker, at least as bosses.
You know how demons have different races? Like Kishin or Snake. These are basically classes. If you fuse a demon of race Snake with the correct elemental, it will become the next, stronger Snake demon. I never bothered with that,except in rare cases, or when filling out the compendium (which might have been the only rare case).
Mitamas keep your demon the same, but add some stats, I think. They also can be helpful, if you want to get an ability onto a demon you already have.
You can fuse elementals, by fusing two demons of the same race. And you can fuse Mitamas by fusing elementals.
None of that is necessary. I always ignored that stuff, and got by fine. It's way more important to cover all your bases (read: elementel spells) and healing. Plus resistances and such. Just keep doing what you are doing, fuse stuff, and if you get stuck on a boss, fuse more specific. You played a Persona, you know all that. You'll be fine.
Also, iirc, it's better to make your dude either physical or magical, not a generalist. He can get insanely strong that way.
Also, I have no memory of these life stone chests, so you can just ignore them. Whatever they offer is not essential.
When you find some machine that lets you play Sokoban, play it through. Use a guide, if necessary. Trust me, it's worth it.