This is not too hard. When it first starts out, it is a game of chicken. He will charge you, you will charge him on horseback and attack with your sword. If you hit, he will fall to the ground, so go in and attack. When he gets back up, he will fly high into the air. When you hear him yell, boost because he is about to land on you. If it hits, you will be thrown off and he will go back to charging, so just repeat. I didn’t notice the attack symbol above his head until I got knocked off Ruin, so I’m not sure how long it takes to get to that point.
For the next part, I just went all out and landed a few attacks. I rushed out and rushed in mostly using air combos, so some of his attacks would miss. You can also go to chaos form if needed.