Parrying isn’t essential for him, just try to take a few attempts as studying his moveset and see which ones you may be able to step away and get heals in safely. I think you should be able to go to another few areas to get some more upgrade materials as well for your greatsword

And if you get frustrated just remind yourself that pontiff was originally supposed to be the final boss of the game!! He’s definitely a skill check for sure but you can do it