I think I died maybe once in the entire game before the final boss, and now the final phase is so difficult I can't even imagine what combination of gear and strategy would let me survive it. By far the biggest problem is that there is no way to gain extra hearts. Every single dungeon run, I would have like 6 extra hearts gained throughout the course of it, so that by the time I reached the boss, it was trivial to beat it. If hearts had been as limited throughout the entirety of the game as they are in the final boss, I wouldn't have been able to reach the final boss without achieving a deep understanding of the mechanics and necessary muscle memory. Instead, I have to learn that while banging my head against a brick wall that takes away 10 faith every time I fail, which is basically like the dark souls 2 health system, where you aren't even free to bang on the wall endlessly, you have to interrupt your skill improvement grind to do some busywork every now and then.

TLDR: either make the rest of the game harder to match the final boss more closely, or put a dungeon before the final boss like there is before every single other boss in the game so that the player can build up the same tools before the fight