Snake Eyes Shirahagi is a non-optional mini-boss in Sekiro: Shadows Die Twice. Located in the Ashina Depths near both the Poison Pool and Ashina Depths Sculptor’s Idols, she’s basically identical to Snake Eyes Shirafuji who you took on in the Sunken Valley. The main difference is that there are a number of enemies armed with heavy artillery nearby, so a touch of preparation and the correct approach are essential.
Start from the Poison Pool Sculptor’s Idol, grapple up to the ledge right next to it, and then jump up to the next ledge up. There will be a gunner stood with their back to you, looking in the direction where Snake Eyes Shirahagi is stood, so taking out this enemy is key. Kill him with a stealth deathblow and then backtrack to the Poison Pool Sculptor’s Idol, jump onto the ledge along the side of the pool and then grapple to the next island over. Walk up to the back of the angled buddha statue and you’ll be positioned to get behind Snake Eyes Shirahagi.
Now that you’re in position, sneak up behind Snake Eyes Shirahagi and wait for the patrolling enemy that’s on the central island to have his back to you. Once he does, hit a stealth deathblow on Snake Eyes Shirahagi and then sprint straight back behind the buddha statue and wait for her to come to you. Taking her on here will prevent you from attracting any more unwanted attention that would make the fight much more difficult.
From here on out it’s pretty much a straight fight, and it’s all about Deflecting and Dodging. Snake Eyes Shirahagi will swing her gun at you several times and will often go for a kick before stepping back and firing a shot at you. Deflect all of the strikes (including the kick) and then Step Dodge out of the way of the shot and get a hit or two of your own in. If you want you can use the Loaded Umbrella to block the shot, but I personally think it’s much easier to just Step Dodge out of the way, and it allows you to get a hit in quicker.
The only other attack to watch out for is her Perilous/Red kanji attack, where she spins around backwards and tries to hook you in with her gun before shooting you backwards. If she goes for this move just Step Dodge backwards to avoid it and then move back in for a hit of your own, then get ready to start Deflecting again.
And that’s really all there is to it. Keep at this and you’ll open her up for the second Deathblow pretty quickly.
Killing her will earn you another Prayer Bead and disperse the fog that was blocking the passage behind her.