I'm sorry. The Executioner's Chariot is easily the toughest boss in the entire game.
Hahahaha....yeah its my nemesis as well.......the killer was that every time I tried to turtle up in the cubby holes I couldn't use the triangle button to switch to put up my shield from two handing because some ASSHOLES YOU KNOW WHO YOU ARE...put messages in there, and together with bloodstains all I would do is toggle through the option to read the friggin messages and watch the bloodstain animation.....couldnt have been more annoyed. I can solo sentinels with no shield so eventually just did the same on chariot by rolling continuously while in the cubby hole!
Heart Cooks Brain does an excellent job editing her version of the Chariot boss here and shares our pain.