Offense
Don't spam the attack button. When you just mash the attack button, you get caught in your own attack animation, and can't do anything until the attack animation is complete, or until you get hit. Plan your sword swings. Like say you plan on swinging your sword 3x, then swing 3x and don't mash the attack. When you're done with the swings, give yourself a split second to figure out your next move by watching the enemy's patterns and how they reacted to your previous 3 (or 2) swings. This will allow you to parry, dodge, or jump out of the way.
You can only plan your swings after knowing the boss patterns, but even when fighting a new enemy, the key is to not swing wildly. Getting greedy with your attacks will just mean you'll get hit. This strategy also works when something unexpected happens. I just had a run against a boss where I was able to swing my sword 5x in a row and is very unusual (see video below). I did it because I wasn't spamming and gave myself a split second to see what the next move was.
If a boss has a weapon and can do thrust or sweep perilous attacks, the best way to determine whether you should jump or mikiri is by looking for the blue flash of light on their weapon. The positioning of that light will tell you exactly what the attack is. You don't need to read the animation. The best part about looking for the light is that you can see the light even through dust and smoke kicked up by combat. The light is designed for easy read on perilous attacks. See the pictures.
This is just for one boss, but the rule applies to everyone with a bladed weapon. Notice the second image has tons of dust obscuring the boss, but you can see the blue light even through the dust.
Defense
Don't over equip yourself. You can actually use items by pausing the game. Equip as few items as possible so you don't waste precious seconds scrolling through. Also, sometimes you might forget what you have equipped in the heat of battle. You think you're healing but forgot you switched to something else earlier and end up using the wrong thing. I usually just equip maybe 2 healing items and 1 extra item. Hell, I've equipped just the gourd before and nothing else, and it turned out fine.
Learn to fight the camera. Unfortunately, the camera is god awful in this game. So try not to get trapped by the environment. The minute you notice your camera feels too close to Sekiro, you're close to the walls. So learn your arena as well.
First of all, you'll notice I was able to respond to every single one of her attacks. Part of the reason is that I know her moves. The other is because I wasn't spamming the attack button. I didn't miss a single perilous attack.
Notice the rhythm of the attacks were either 1, 2, deflect or 1, 2, 3, deflect. I don't know what she's going to do, but I leave myself with a little room to react.
You'll notice I try to back up whenever she's high up. That's to get a better camera angle. I got lucky in this fight and she didn't jump too far causing any camera issues.
Around 1:57 mark I managed to swing my sword 5x. While I know her moves, this is completely unexpected. I reacted like it was natural because once again, I wasn't spamming the attack button.
Around 2:09 I made a mistake and got hit. I saw the attack and knew what was coming, I couldn't react because I was in the middle of my attack animation. This is the only real hit she scored on me. The other damage came from her unblockable butterflies.
Not all fights will play out like this with an aggressive back and forth. Some fights require a combination of deflecting and waiting for an opening. Some can be fought completely with deflects. There even the occasional boss where doing vitality damage works better than deflecting.
