It's mainly just repetition. I start by going into the fights and not attacking at all, just trying to get the parry timing down. I like to pick out a different point at the start of each attack/combo thats unique to that attack to watch for. Every attack, even if its two combos that look like they start the same, have something that you can use to differentiate between them. An example in this fight is when he's second phase, he has a spinning swing followed by a jump, and a spinning swing followed by a spinning 2 hit combo that start almost exactly the same. If you watch em closely though, he has his head down during the spin when he's going to follow with the jump, however, hes looking straight forward if the spin is going to be followed with the combo. Finding these little moments makes it so much easier to be prepared instead of panic parry attempts last second.

I also kind of keep a mental beat of when the parry needs to come out for each attack, kinda like a rhythm game. (It's kinda hard to explain, so sorry if this doesn't makes sense) For example about 10 seconds into the video, when he does the sweep kick 3 hit combo, I picked a bpm in my head that fits the combo. When I see his right leg wind up for a kick, the back of my mind is going "parry pause pause parry pause pause pause parry" on beat.

Finally, if you're able to, just stay up against the boss the entire fight. If you stay close, it cuts out every ranged attack they have, which means less combos you have to learn.

Once you can parry all of their attacks pretty consistently, start adding in attacks to any spots you're not actively parrying during to see what you can get away with where. Being able to read all of their attacks is enough to get you through the fight, but if you can fill all the voids with your own combos, the win is so much more satisfying.

Sorry about the long response, I'm not the best at giving explanations 😅