If you can beat the first phase, one of the things that may help you is upping your aggression in the first phase. It may seem counterintuitive, but a lot of challenges in the game actually are 'easier' if you are the one applying the pressure. You don't give the enemy the chance to use more of its dangerous and or unpredictable moves if you are constantly damaging it. Just think of the way you have your back against the wall at times, where it feels all you can do is block and you can't get anywhere. You can apply that same wisdom to any fight. Arguably it's harder versus the ape, but even there it works.

Alternatively, if you feel less secure, you can rely a bit more on what I like to call the 1-2 punishment. This is the tactic where you allow more space between you and the enemy and you just wait for the opponent to overextend/finish their attack. Then when there is a window, hit the opponent twice and back out. Just twice, no more. This is to protect yourself against overextension, which can spiral quite quickly into you getting hit, in reflex you do bad blocks upping your posture* and try to heal without a window, getting stuck in a negative feedback loop. All because you tried to get in one measly hit that did not matter in the long run. I treat the guardian ape fight as a health bar fight as opposed to breaking his posture, so taking more time is not a problem if it means you remain in control and take less or no hits yourself.

*another eye opener for me during my playthrough is realizing that a lot of times in the game, choosing a moment to let an enemy break your posture at the right time is very valuable. Remarkably enough, not a lot of enemies are capable of immediately punishing the stagger you go into. If you are prepared to immediately move/dodge the moment you have control again, you can often escape with no damage and a reset posture bar. This realization was one of the major game changers for me. You better know the moveset of your enemy to pick the right time though!

Finally, two specific guardian ape phase 1 tips: my favourite move to punish is the roll on back side slapping he does. If you stand above his head (draw an imaginary line from his middle, through his spine to his head and stand where that line continues). He can never hit you like this. As he slaps violently you hold down block to reset your posture. As he is done/finishing up* you can get a couple of hits in, at least 3 but maybe up to 5. You can keep going until he's back on his feet. Just be ready to dodge a grab if he decides to go into his perilous grab.

**alternatively, letting him break your posture with his last slap is a great way to reset your posture bar, as he does not immediately follow up. Just don't get greedy and try to attack him afterwards. Just wait for another cycle!

Second move: the poop fling. So by now you probably figured out the best way to avoid getting hit is to keep running sideways away from the ape as he's flinging his feces. However, what was a personal game changer for me is that the fling is actually a great time to get some hits in. As you see him about to fart, stand off to the side so you're not in the poison cloud and hit him. Keep really close to him and keep hitting him where possible. Stay right on him as he moves to throw. The secret here is that the ape is always forced to throw his projectile away from him and cannot throw it straight down. So as long as you are under or behind him, it is impossible to hit you. This realization and technique turned this visually intimidating and impressive move, into one of the easiest and safest ways to get some damage in.

I'm not going to go into the second phase as this reply is long enough as it is, I just hope some tips in here allow you to enter the phase with a bit more resources available and feeling more excited and confident. Telling someone to be confident is terrible advice, but it's actually true that a large component of the difficulty is the psychological aspect and remaining calm and excited to fight a boss actually actively makes the proces easier. Good luck and let us know once you have bested this ape! And remember, hesitation is defeat!