For those of you that are struggling, ignore the asinine "you're just bad lmao" people. Here's some actual advice from someone who was in your position very very recently.
1. Guarding recovers posture faster, keep your guard up when the enemy is at a distance
2. Keep the pressure up, there are very few times in this fight when you want to run anywhere but toward him, never let his posture meter go down more than a centimeter.
3. Use your block to find patterns in the combos, once you're semi-comfortable with the rythm, start deflecting.
4. Deflecting is almost always better than guarding, do not hesitate, or wait until you suddenly get better at it. Practice it, even if it gets you killed, you'll need this skill.
5. If you don't already have it, get mikiri counter, this halves his effective perilous moves and gives you good posture damage
6. When you see a perilous, look for either his body movements, or a flash of white light from his blade, the position of these will always tell you what is about to happen.
7. Optimize opportinities to deflect, not attack. If you see him go for an easy move? Let him do it rather than clashing blades. See him stop swinging against you? Chances are he's about to try and jump, so get some slashes in and you know what's coming.
8. If all else fails, or you get frustrated with the second phase and wish you could just practice that instead, remember that people around the world have done this and posted it to youtube. Observe, Internalize, Preform. This will spoil you to some fights though, so it should be a last resort.
If anyone has something to add to this, feel free to. I only just beat him myself so I could be unaware of something important about the game. I've just learned to recognize patterns from my time in souls. I guess that's a hidden 9th tip, if you're from a souls game, stop trying to play sekiro like it's turn based. Fight like you're fat rolling in full havel's with a bandit knife and a buckler, fear nothing.