I don't know if I've ever truly appreciated a final boss more than this one. I feel like it's a big trope in games for there to be some secret boss you have to seek out that is harder than anything else in the game. Not just Metroidvanias and recursive/Souls-likes, but RPG's as well (looking at YOU, Final Fantasy). Not the case here, nothing else in this game even comes close to the final fight. Lady Ethereal and Fuxi and Nuwa were both difficult, but I was able to get through both in 1-2 hours. And I suck at parrying, which is ESSENTIAL in this game. True Eigong took me right at around 6 hours to beat. What sets this apart is that is that every skill you obtain, except maybe double jump (though I used it regularly) is needed and put to the test here. You could get by without the charged parry too, but hitting those is a great way to get a hit in or heal. She uses the entire gamut of move types in rapid succession, whereas every other boss focused on a few. On top of that she really punishes you when you don't parry her attacks. The game has a mechanic called internal damage, which is similar to Bloodborne in that the damage shows as faded red. This comes from imperfect parries. You can recover all of this over time or with buffs that reward attacking. That being said, if you take actual damage or get hit with a talisman, it turns into real damage. With her constant barrages, it's real easy to build up a lot of internal damage, and then mistime something, leading to massive damage. That being said, it goes both ways, though it's more like chip damage to her. I had a buff where all perfect parries did internal damage to her, and then I would use the auto talisman to convert to real damage. Auto does less damage overall, but it's a set it and forget it type of deal, normally you have hold the button and choose when to detonate, leaving yourself open to have it cancelled. The other thing I really appreciated about this fight is that there doesn't seem to be any way to cheese it or make it much easier beyond what upgrades are in the game. Nothing broken or overpowered from you, it's just learning the patterns and being able to react appropriately.
Found this video of the fight, which is the closest to how I played it. Only real difference is I used a different arrow, and usually jumped over the first red sword slash that kicks off the second phase. You have to do the charge parry on red attacks, and I was struggling on the timing for that one.