This is NOT an easy fight.
You have to learn the boss move set. There is no second choice. No cheese or butter.
For Phase 1:
Keep hitting him and don't wait for anything.
Learn the move set, parry the attacks and use R1 + Triangle (On PS5) whenever possible to get that Red meter going up on the boss. Once full, press Triangle for the Big blow.
For Phase 2:
You'll only need to get his health down to 50%.
Again, learn the move set, keep your distance and block/parry the usual attacks.
For the unstoppable Red attack, parry it and keep hitting the boss untill his gives you the 'Red Dot'. Then press Triangle to give the BIG blow. You'll need to do this 2 to 3 times to get his health to 50%.
Once down to 50%, press Triangle and Circle and its all over.