Discuss FromSoftware's Sekiro: Shadows Die Twice
Visit the Sekiro Shadows Die Twice Wiki
If I try fighting her in the poison, I get slowed and take damage over time; If I hide to lure her back on land, she regains all of her posture. The boss is rewarded for being stupid and standing in the poison.
Might as well abuse that stupidity by letting the poison finish her.
Gg I can kill her blind folded.
Having beaten every other Soulsborne game, I can say with reasonable certainty that this is the most annoying are in any one. Seemingly impossible to kill anyone else without pulling the miniboss, seemingly impossible to reset once you have...
The charged spear can be used to rush her down and pull her back to you. Deals high poise damage and lets you stand close for a sword trade. just beware of her grab and its very, very doable and fast
If you kill the guardian ape first, he'll head down here and smash up the place, so they'll only be two canon wielders around the boss, both of which can be stealth killed before she notices you.

I'm really surprised this isn't on this page, unless I missed it
This boss is an insult to fun.
This entry is misleading. She’s not immune to poison. I got her down to her first deathblow by accident when I hid on a stand to heal after killing the mob enemies; she walks right into the bog and poisons herself. Set the controller down, make a sandwich, come back and shuriken her for the deathblow, then go back to hiding.