@Nightweaver20xx
do you have Power Within? it’s a pyromancy spell.
also,
check this part in the video. I think it was the 300 damage mark that did it, the other one dealt more damage and as my memory+wiki notes serve, only while he’s buffing he has a good deal of damage reduction from your hits. That’s the part that matters most in this fight, as if he buffs, it’s like he becomes a different boss fight I guess.
and another thing that people said interrupts the channeling of that buff, is a headshot from a ranged weapon.
but if you have Power within, from what I’m hearing here that’s what you need, the slow health drain overtime is obviously bad, but the damage you deal as a result should make the difference here.
as for a summon sign, I could try, but I got O&S on NG+ and I’d need to know your level, so that I’m not too overleveled..