Isshin, the Sword Saint is the most retarded boss fight ever. Honestly beyond my level of autism.
I've spent 2 hours on this shit, got to phase 3 a couple of times but then I died with almost all my Gourds.
The safest way to go about it is by being ultra aggressive against Genichero and Sword Saint phase 1, you want to lose no more then 1-2 health pots during this period and if you're struggling to do that just focus on practicing on those.
If you can comfortably make it to phase 3, the key is to change your play style and be far more defensive which can feel like doing a 180 but is necessary if you want to avoid too much damage. In this phase you want to bait out the big jump attack he does and the attack which ends with a thrust so you can mikami counter (honestly, mikami countering him is very easy since you're waiting for a long attack to finish, just make sure you press the button while not pressing on any of the analogue sticks and wait until the thrust attack is actually happening, don't just press it as soon as the "red" thing appears).
If you block the jump attack, I found that 50% of the time he'd follow up with a one of thrust which you could mikami counter, these are the best sources for doing posture damage, just ensure you're also getting a hit into his health bar when you do because you want to wear down his health where you can so that playing defensively works, if you dont' do this you'll be at it a while because high health means his posture will recover quicker.
Finally, with the last phase I'd advise just playing it safe if you're having trouble. You CAN counter his lightning attack by jumping and pressing R1 (or your button for attack) at the right time but it has its risks despite being very, very effective so you might be better off just dodging, same with when he flings those sword attacks at you, just dodge to the left and you'll be fine.
Easily the toughest fight in the game but it just does come down to doing the opposite of what you did in the first 2 phases to make it manageable, once I started doing that it only took a few tries.
EDIT: Also when you do make a mistake against phase 3/4 you need to be really careful with when you heal, if you heal at the wrong time you'll end up wasting 2-3 health pots after he counter attacks you which can kill the run, even if you take damage don't panic and try and heal straight away, wait until he's in the midst of an attack. Some of this stuff is probably common knowledge but still worth mentioning.