Are you talking about
?
Yes, he does tons of damage. This is one of those fights where making use of regain can be really helpful. While he might take out a third or half of your health, you can often get close enough to hit him repeatedly and reclaim most of the damage you were just dealt.
It can feel really risky, and it kind of is, because you are constantly trading damage with him. But it's enough to keep you alive if your weapon has strong rally potential.
Other than that, unlocking and just running away as fast as you can is a very effective strategy. Once you are at a big distance, you have time to heal, and regain your footing. He can actually be really easy to "approach" because it is easy to react to his attacks when you have lots of room between you two.
I also usually hate this advice, but "getting under him" can also be really helpful. He can't reach you under there, and with a big weapon you have lots of room to connect. If you can get him against a wall, he won't have anywhere to go, and you can just run out your stamina meter. Usually he will start trashing and stomping, which will do lots of damage, but it also has a long animation.
So you can let him stomp you, but then run away and heal up. Then reapproach.
I used LHB buffed with a Phantasm Shell for this fight. Fire Paper did more damage, but I was failing too often to keep using it on every retry.
This will get you through the first half of the fight.
The second half is obviously way different. His attacks are even more powerful and can be even harder to dodge, but they're also much slower and much more plainly telegraphed. Once again, putting tons of distance between you and the boss will give you lots or room to approach. You can dodge his attacks more easily this way and damage him during his recovery animation.
Sometimes he will lunge forward, which leaves him open for a comparatively long time and enables you to approach from behind. Just wail on him here. This is usually your "big opening."
I didn't finish the fight gracefully by any means. I was consuming Blood Vials ravenously and burned through hundreds of them in my constant reattempts. I also never figured out how to predictably evade him when he jumps up and drops down from the ceiling. But I did get better every time, and eventually it worked.
