Keep a medium distance from him (far enough from his reach but not far enough that you can't reach him in under two seconds), wait until he does one of his jump attacks and rush him as soon as he lands, use firecrackers to stun him and then hit as many times as you can get away with, then regain your distance and wait for him to jump and land again

Jump to avoid his grab attacks, it's telegraphed by the red kanji sign popping up

In his second phase he has three unblockables; The scream (run away as soon as you see him setting it up), his jump attack (pretty much the same thing from his first phase, except don't rush him, the name of phase two is parry, parry, parry, his sword is your friend) And finally the swipe attack, just jump as soon as you see the red sign

In phase two you want to parry him until one move where he stands tall and rises his sword way up in the air and smashes it down, don't get intimidated it's very easy to parry (click the parry button as soon as you see the sword start to descend) and when you do, it stuns him and takes a chunk of his posture, as soon as he's stunned in front of you try to use the loaded spear and hit him in his severed neck, it does incredible posture damage.....after that continue dodging his unblockables until he gets in a parrying mood again and brings out that crucial sword rise attack

I literally just beat the two ape guardians an hour ago for the first time, just use the second phase tips from the first ape guardian, they're identical, but when he brings in his friend concentrate on the brown ape, to do so you have to hit the headless ape once to trigger his scream, run in there and tap the headless ape and then make some distance from the scream, while the headless ape is busy screaming the brown ape will chase you, before he reaches you spam the firecrackers, it stuns him hard, then mash the attack button and get as many hits in as possible before the headless ape recovers from his scream and reaches you, rinse and repeat around three times to kill the second ape and finish off the headless ape the same way you did the first time