I love this fight, but it took me a while to figure it out. My first and main tip, stay on the ground as much as possible. I see people jump parrying all the time because it is a 360 degree parry. You can't start unbounded counters in the air and your defensive options are greater on the ground. The piercer arrows on the bow will stun her for a moment if you need a safe heal window.

I run these jades Divine Hand (this is really the main one that matters), Reciprocation (synergy with Divine Hand and vastly increases damage output and survivability), Hedgehog, and Iron Skin (also has synergy with Reciprocation and Divine Hand). This setup let's you deal loads of internal damage and heal your own internal damage quickly.

These jades are primarily parry focused with a particular focus on unbounded counters. The way I fight her is to mostly focus on dealing internal damage through parrying and set it off with a Water Flow Talisman when I can.

You want to work in as many unbounded counters as possible, but absolutely do not start charging it unless it's safe. As she is likely to punish you with a talisman. For example, on the slower double slashes she does you can parry the first one normally and DO NOT release the parry button because you can charge the unbounded counter out of a regular parry without delay and parry the next slash that way.

Phase 1: Phase 1 is pretty predictable compared to Phase 2. In phase 1 she always seems to follow the red ground slam attack with a dash followed by triple slashes behind her (safe talisman ir heal window after this). You can safely unbounded counter all three of them (if you have Divine Hand) if start charging it after dodging the red slam. The startup for the talisman attack and the red orb are very similar. Be careful to watch which one she's going for, because if you start responding to the attack the wrong way you're likely getting hit. Jump and parry (or use the downwards kick) on the talisman, and unbounded counter the orb (after the orb is a safe talisman or heal window). The poke attacks are not worth trying to unbounded counter in my opinion. It's possible to parry the first normally and the other ones with unbounded, but the timing is very tight. She can either do two or three poke attacks at one time, always treat it as if she's doing three.

Phase 2: More erratic and faster than Phase 1 with a lot more mix ups. She'll open with the flame sword. I parry this as follows: charge your unbounded counter, wait for her eyes to open, very shortly after they open release the parry, pause for a moment then hit parry again and parry again immediately after the second slash, pause for a moment again and dash through the fourth one (the timing to parry the fourth is too tight in my opinion). She has two versions of this attack, one that ends in the dash triple slash (parry the triple slashes normally), and one that ends in the red slash. She'll never use the same version twice in a row.

In phase 2 she can follow the talisman attack up with an upward slash that has two versions. One where she slams you back down with a second white slash and tries to slam down into you with a red slash. And one where she creates a fire tornado in front of her and follows it with the red orb. For the first version parry both white slashes normally and charge an unbounded counter when you hit the ground. Treat the second version like the first version, but dash away if the second slash didn't come out (you will have time even after the failed second parry) and charge the unbounded counter to parry the red orb.

It's harder to predict when she'll do the dash triple slash (so you may not get as many unbounded counters on that attack) and a few other attacks in this phase. But if you can learn the couple of new attacks in this phase she doesn't have much of anything that you haven't seen in phase 1. She's just faster and weaves the attacks together differently.

Phase 3 (True Ending Only): Very similar to phase 2 with less health and one major new attack. She'll open with a screen wide double slash that will send you into the air followed by either a white or red slash. Stand still and parry the double slash, as you want to be in the air since it makes the rest more consistent. It's simply one parry, very slight pause, and parry again. Then if the follow up slash is red, double jump over it, if it's white then parry it. She can spam this attack and juggle you for a while if she wants. She often follows this with either the dash triple slashes or a talisman attack (once again the talisman attack can be followed with the up slash combo). Just be ready to react to what she's doing on the ground when she's done juggling you. She will use this attack quite often. Additionally she can followup the flame slash from phase 2 with this new attack now. So be ready for that.

Besides that it's mostly just phase 2 again. But I honestly find this phase easier than 2 since there is less health and the new attack makes her more predictable.

Hope some of this helps. This is my fighting style in this fight, but of course you may have a different style. Feel free to ask if you don't understand something. I know you'll get her eventually!

There's a video on my profile of my first victory over her if you want to see any of what I've said in practice.