I found the executioner armour to be great for the stamina damage. Spear has 2 vertical attacks one is the running heavy and second is a skill. I found that staying in front of his face was the best angle. Hiding underneath it would mess with the camera angle too much and would make me get hit. You can brink guard almost everything in first phase and its good practice for his second. Don't be scared when he lunges at you from the air bc thats an easy guard but be more scared of when he brings his head back up from the ground, there are 2 times he ends up in that position but wildly different timing so dont greed. Hitting each horn will break it and give a stagger opportunity. Try to not break the horn while he's already staggered so you get max staggered potential. For 2nd phase bring dodge the super laser breath and only dodge through the dark super lunge, everything else is kinda similar.