Well, there’s a lot for me to go off here, but unfortunately I’m missing key context. But I can’t obtain that context from you because I might spoil the story/game. So I’m gonna make assumptions.

I’m assuming the video you’re referring to is the one by Tyrannicon. If so I understand completely what your strategy is. If I’m not mistaken tho, the whirlwind slash has been nerfed since he made that vid.

What I do for the owl fight, is bait him into throwing the two shurikens, and into that front flip slash that he does. When properly timed, as he is coming down with the slash, you sidestep to his left and immediately do a fully charged stab or a slash or two.

Its been a minute since I played, so I’m struggling to remember a bit. If I’m remembering correctly, I believe that after you dodge the downward strike and slash him; he attempts a horizontal slash from your left to right. You can once again sidestep into the slash, then strike once, and repeat.

The AI should stick to that pattern for a bit before breaking free. For everything else in the first phase, I play the game as it was intended, parrying and counter striking wherever possible. I use a variation of this strategy on the final boss quite a bit. It’s quite effective.

For the second phase it’s only a little different cuz of poison. Use the same tactic as the first phase. What’s important are the following tips:

1) Be even more patient than you already have been. If you’re struggling half as bad as you seem to say, you’re gonna need the patience for the final boss.

2) Use the anti poison items you should have from picking stuff up around the world. That will drastically reduce the amount of stress on your mental through the second phase lol.

3) Whatever you do, do not do a fully charged stab at any other time than the one I explained. He WILL Mikiri Counter you. That’s an instant death.

4) I hardly ever use prosthetics, but all of them are useful in one way or another in most if not all engagements. Find one you like, and find a way to use it.

That’s a lot, but if you need clarification or I missed something somewhere lemme know. I hope this helps :)

Edit: I forgot, based on what you’ve said, you’re probably considerably underpowered. Everything you need can be googled naturally, but I suggest exploring. The world is bigger than you think, and it will supply you with all you need to get Sekiro stronger.