The Guardian Ape is one of the later bosses encountered in Sekiro: Shadows Die Twice and can be found in the Bodhisattva Valley located in the Sunken Valley. The door to access him is located behind the Centipede Giraffe boss but you'll need the Gun Fort Shrine Key from Kuro's library in order to unlock it and gain access to him.
Guardian Ape strategy
The Guardian Ape has two phases and is unable to block any of your attacks. This doesn't mean he's a pushover though; he spends a lot of time flailing his long arms or trying to crush you by rolling around, so you'll need to be quick to deflect or dodge his attacks. Firecrackers can be useful to stun him during the first phase of the fight but the second phase will require a lot of patience as he gains a whole new move-set.
You fight the Guardian Ape in a large, water-logged area. Despite all of the space available, you want to stay as close—and to the back of him—as much as you can. Many of his attacks can be deflected and you'll want to be as aggressive as possible in order to stagger him, which will allow you to get a number of attacks in before he runs away. The Firecracker prosthetic is useful to stun him and get off a few valuable hits but use them sparingly as they won't work if you just spam them.
He'll also (rather charmingly) throw his faeces at you, inflicting poison, so be prepared to dodge if he backs away a fair distance and reaches behind him. He'll also occasionally 'release' a poisonous gas when you attack him in the rear, so it's advisable to keep some antidotes handy if you get hit.
You'll want to look out for his grapple attacks as they will deal massive damage; the first is a sweep attack that will give you a red icon over your head, prompting you to jump to avoid it, the second is a charging attack where he will run at you, ending in a belly flop. If you can avoid both of these attacks, keep chipping away at his health and he'll soon drop.
Phase 1 will end when you execute the Guardian Ape by taking off his head.
The second phase of this fight arguably harder than the first. When the Guardian Ape regains his feet, he is now equipped with the sword that you dislodged during his execution. He'll also gain a whole new move-set, along with a much larger attack range.
You'll want to keep your distance for much of this phase; most of his sword swings can be blocked or deflected but it's best to stay as far away from him as possible. He'll occasionally cast a 'red terror' fog that inflicts fear which you want to avoid at all costs. If your terror meter fills up completely, you'll die, so Pacifying Agent is handy here if you do get caught in it.
The ideal time to get some hits in is following his overhead sword attack. He'll hold the sword high in the air before bringing it down towards you. If you can manage to deflect this attack, you'll stagger him, giving you the chance to get a number of your own attacks in. Another opportunity to deal some damage is when he lunges forward to take a huge horizontal swing at you; you can jump over him and get a few hits in before backing away to safety again.
If you can be patient, keep your distance, and only attack when there's a clear opening, you'll soon take this boss down.
If you're still having trouble, check out our handy combat tips for Sekiro: Shadows Die Twice.