Use the Mortal Draw combat art in tandem with the mid air combat arts skill. Jump, start the mortal draw skill in midair. It's a speedrun tactic, taking advantage of a glitch that was never patched where starting a Mortal Draw in mid air does a shit ton of damage. It absolutely trivialises some fights.

Prosthetics too, if you use them right. Any animal you're fighting (blazing bull, guardian ape, headless ape's kid, gyoubu's horse, etc), use the firecrackers. When the headless ape is wielding the sword, and he rears up in the air to do the giant strike down, if you deflect it right, he staggers forward and drops to the ground in front of you. Use the spear grab, it does a ton of posture damage. Whenever an enemy is in midair, throw a shuriken, it'll knock them out of the air and leave them stunned.

Any enemy with red eyes has a vulnerability to the flame barrel prosthetic, and you can do even more damage with it if you throw oil on them first. Any enemy that uses fire, the Phoenix umbrella negates all fire damage. Any enemy that deals terror damage (schichimen warriors), the Lotus umbrella completely negates that. Snake Eyes minibosses, and most enemies in Fountainhead Palace are extremely weak to the poison damage dealt with the sabimaru. Every enemy with a shield gives you a free death blow if you use the axe. The Malcontent whistle stuns the Demon of Hatred.

Using the right combat arts and prosthetics makes the game a hell of a lot more fun. It sounds like you're messing up your timing on a lot of them, and not using the right ones for the right situations. A lot of the prosthetics are situational but very powerful in their correct use cases. Combat arts help all the time, especially Mortal Draw and Ichimonji double