Chained Ogre
The first boss you will encounter in Sekiro. This boss uses a multitude of grabs and wrestler type attacks that hit like a truck. The best way to dispatch this enemy is to stay close and use you dodge to avoid his grabs and smashes. Also, if you have the flamethrower tool on your arm, use it with oil to cause a large stagger, creating an opening for lots of hits. Be aware that many of his grabs have tracking so you will need to time your dodges.
Gyoubu the Demon
Gyoubu fights you on a horse and uses a spear to smack you as he rides around the arena. For this fight, you will want to have the Shinobi firecrackers on your arm to stun the horse for free hits. Make sure you stay close to him throughout the whole fight and if he does get away, use your grappling hook when prompted to close the gap and get free hits.
Genichiro
Genichiro is a swordsman who also wields a bow so most of his damage is through sword combos and jumping bow attacks. Focus on deflecting his first attacks and counter attacking immediately. The best way to win this fight is to stay aggressive and give him no time to recover. When he enters his second form, make sure to jump counter his lightning attacks and send it back with R1 for massive damage to posture and health. He will also use his bow many times in the fight and its best to simply hold block for these as deflecting them will not lower his posture.
Guardian Ape
The Guardian Ape has attacks that are not very telegraphed and therefore it is advised that you don't play too aggressive but rather only get a hit or two in at a time. Both the flamethrower and the firecracker work wonders to stun him for free damage. He has two grab attacks, the first can be jumped over while the jumping grab must be run from as it will track your dodge. Once he enters phase two, deflect his sword swings and when he does the large overhead swing, deflect it for a long time and free damage.
Lady Butterfly
Lady Butterfly can be beaten by attacking continuously until you are deflected and then immediately blocking. Anytime she tries to jump into the air, make sure to use shurikens to bring her back down. In second phase, make sure to use snap seeds to get rid of the illusions and run from her long attack combo as it will probably break your posture. When the phase begins, make sure to rush her and hit her asap as it will stop her from summoning the illusions right away.
The Owl
The Owl can be beaten by using the general concept of attack until deflected and then holding block until you deflect him, then following it up with your own counter attack. He will use a lot of the same shinobi tools you have as well such as the firecracker and shurikens. For the firecracker attack, watch the air and when you see him toss soot into the air, dodge away. When he jumps off of you into the air, hold block as he will throw shurikens at you that need to be blocked. Finally, he will try to use a ball item that he tosses into the air and when it hits the ground, it will explode into a green mist that cancels healing. Make sure to avoid this.
The Divine Dragon
A very easy boss fight. Simply wait for the trees that spawn to have lighting over them and then grapple up to them to gain the lighting power. Proceed to r1 that lighting back at the dragon for massive posture damage. Continue this until he is defeated. He will occasionally knock you back with wind and the proceed to throw wind attacks at you, simply dodge left and right and then jump when you see the red symbols appear.
Corrupted Monk
He has many wide swinging attacks that you can deflect in succession for lots of posture damage. Make sure to keep up the pressure and mikiri counter his thrust at the end of his combos. Occasionally, he will use a sweep attacks as a starter which needs to be jumped. In his second phase, make sure to use the trees around the bridge to avoid his phantom phase and then in phase three, make sure to watch out for the red symbol attacks as now he has a fear spit attack mixxed in that you will need to react dodge.
Sword Saint Isshin
The hardest boss and final boss in Sekiro. Don't let yourself get frustrated as he will take you a lot of time to beat but rest assured, you can do it. The first phase is fighting Genichiro so you already know how to do this, just do everything the same and he will go down. Ishinn's first phase can be easily beaten by using the attack until countered and then deflect + counter strike mentality. After you counter strike him, he will either fall back into a hold stance of thrust attack you. For the hold stance simply block and then jump counter and for the thrust, use mikiri counter. Phase two and three rely on you dodging his overhead slam for free damage to his back while staying aggressive. Don't give him to much distance as he will use a gun if need be to finish you off. The trick to beating him here is no matter where your hp may be, you cannot back down and run away, he will punish you severely for it. Finally, use lighting counters on him in the last phase for strong punishes.