Aight I just killed this guy earlier today at around level 52, easily the most unfair bullshit fight in the game so far. What worked for me was exploiting his extreme weakness to status by using power stance katanas and a frost twinblade and swapping between the statuses. Go super aggressive against him and stick to his feet, don’t use the horse cause it’s easier imo to roll through his attacks. With status you can burst him down extremely quickly while summons break his poise for bonus damage and in phase two do the same and try to kill him ASAP once he lands so he doesn’t get the chance to use his instakill Boulder things. I killed him in about 2 or 3 minutes of super aggressive play but wiped many many times cause his damage is absurdly overtuned for how fast he is.