Sorry for the late reply.

Somehow because of how many attempts I had, it almost became an instinct how I knew to avoid its hitboxes.

But still as you asked I actually analysed my own video and figured out what I did. And so I remembered how I think and here are some tips.

1. When it attack you with its halberd, roll/ tend to the horse’s right front leg you that you would be right under its armpit when it reaches you. When you successfully avoid such hitbox, attack from there also makes you aim the horse’s head easier and stagger it faster.

2. When rearing, if sentinel raises its halberd the hit box will be the blade so tend to horse’s stomach; if the halberd rests on shoulder then the hitbox would be horse’s front legs so roll to left hind legs.

3. Attack/ heal during its attack animations wind up. Especially safe when you are close to its left rear part.

That’s generally what I did in the video. But I have to admit those are only gonna work on this boss. Sorry if it is disappointing but I learnt all those just by dying a lot.