he can poise break you in one hit with his basic lung at 15 scadu blessing if you are below 40 poise.
Basic lunge? You mean his full on CHARGE? The one that's super easy to dodge and has so much of an opening that you can HEAVY R2 FULL CHARGED on him?
Nah.
Pretty much any video game player logic would dictate that the method where you don't get one shot is the intended method
Nah, there's a million extenuating circumstances involved in any loss. Very few players are able to judge them all effectively and that's on them.
If they're struggling on foot AND ALSO on the horse then a humble and intelligent player would conclude something like
"Oh I need more of those Revered Ashes that make my horse tougher!"
But OP isn't humble nor intelligent. He just stomped his feet and spewed about how UNFAIR the boss is when he's unable to even figure out or apply the intended way to face him.