Plus the game is mostly about execution. Just because you watched a video of someone perfect deflecting all attacks doesn’t mean you will now magical be able to.
The usefulness is in things you didn’t realize you were doing which were counterproductive, or discovering openings for attack you didn’t know existed, or using a particular prosthetic tool you would have never thought to use on your own.
For the last boss there is no real silver bullet though. It’s the final test and you either sink or swim.
For his spear phase I would recommend trying to focus more on deflecting and trying to stay close to him. He excels at the native range of his spear. When you are point blank all up in his grill, he has to resort to using his sword…with his free hand. Mikiri counter is also always your friend. Most of his combos end with a perlious thrust attack.
It’s the last phase where I panicked, hyperventilated, and proceeded to jump off the cliff edge for no apparent reason. The ultimate choke test! (In my defense it was during a lightning reversal attempt)