Great Shinobi Owl
This fight is an awful lot like your earlier fight against Genichiro. Owl uses a mix of long-range and close-range attacks but also has a couple of other tricks up his shinobi sleeves. The usual rules about deflecting everything you can applies here of course, but there are other things to pay attention to.
If he raises his foot into the air you will want to back off as quick as you can. This is a brutal grab that does obscene damage and can’t be blocked. He also occasionally takes a knee before tossing up an item. Run around to the back of him when he does this for a few free hits but don’t let the mist touch you. It stops you using items for 30 seconds which makes it incredibly annoying and potentially lethal.
His second phase involves him using smoke bombs and poison attacks. Whenever he chucks a smoke bomb just run out of the cloud away from him. You should be able to avoid the poison he throws when he backflips by just moving away from him.
Corrupted Monk (True Monk)
Good news, this version of the Monk has three phases instead of one. The first phase is almost identical to the version you have already fought. Parry your way to the first Deathblow and then the... fun starts? We'll call it fun.
Upon entering the second phase a black mist will descend, this is your cue to start grappling around like Spider-Man. This will happen a few times in this phase and signals the appearance of several ghost Monks who are better avoided. If you use the grapple points either side of the bridge this is easy to do, just don’t stop moving.
Phase three reveals one of the creep centipedes and a few new attacks. This is closer to the first phase than the second but he has a new unblockable attack. Upon seeing him raise his arm and a red kanji you want to nip to the back of him to avoid the insectoid spray. It causes terror and will probably kill you. It also leaves him open though, so get hacking. Just keep on top of the other attacks while noting this and he will fall in no time.
Old Dragons of the Tree
Divine Dragons of the Tree
This is definitely one of the easier fights in Sekiro. There will be a horde of small dragons for you to take out, so run around a lot and introduce them to your dash attack. They will spit out poison occasionally but it is easily avoided. They will summon a tree to attack you, but this is easy to spot as you will see it writhing around beneath you before it attacks. Just run in circles until it is over. Brown dragons will eventually appear, try to ignore them and just finish off the white ones to get to the Divine Dragon fight done.
This fight looks far more intimidating than it actually is. The dragon’s sword attacks can be avoided by simply running to the side of the vertical slashes or jumping over the horizontal ones. In order to take out the boss, you want to look around for the electrified trees. Grapple up to these and then use the lightning to attack the boss. The Divine Dragon will fall quickly.
Demon of Hatred
This boss is large and one of the few fights that doesn’t really factor in Posture. The Demon has a huge Posture bar so it will more often than not be better to just slowly whittle down his health. You generally want to be by his leading leg as it allows you to circle to the left and avoid most attacks while also dealing out damage.
The unblockable attacks change as you go through the Deathblows needed to kill him off. Generally speaking, the best way to avoid them is to simply run away from the boss. He will occasionally jump and land causing a wave of fire. Your best bet is to grapple on to him after for some easy hits. Keep an eye out for grapple opportunities throughout the fight as it is always a good chance to get damage in.
The final phase has him firing off fireballs. He launches them in barrages of eight so pay attention to them but make sure you are running in a circle to avoid them. If you head back towards him as he is firing off the last one you should be able to get some more damage in. This is also the phase where being underneath him isn’t as safe, so it requires more patience. Keep an eye on his legs because he may stomp you, he signals this by bringing up one leg, just back off until it is safe.
Genichiro, Way of Tomoe
While this fight is very similar to your earlier encounter with Genichiro. The main difference is you now have his Mortal Blade to watch out for. He will occasionally crouch and charge an attack, move backwards away from him during this as it does huge damage. He can also turn this into a two-hit combo if you are careless so making sure you aren't;t there is paramount. Other than that, he only needs one Deathblow so make good use of your parry and the Mikiri Counter in order to take him out.
Isshin, The Sword Saint
Phase one is a straight sword fight. It is the easiest of the phases in terms of deflecting his attacks. The key things to keep in mind mostly involve timing. He uses peculiar timing to his attacks that mean if you want to get the full deflections, you really need to watch his attacks. There are chances to Mikiri Counter him as well as jump on his head, so keep an eye on his movements. When he sheathes his sword it usually means he is about to dash at you. If you run away from him you should be able to get a free hit in before he fully recovers.
Phase two and three both involve him using a big old spear. This complicates things if you want to parry attacks as his timings are even more messed up and now he has a ridiculous range. There is a trick that works in both phases though. If you run in a large circle around him he will attempt a jump attack. Run under him when he does and use a running attack before running away again. You can whittle away at his health without even worrying about anything else.