How the hell do you get past myrkul on honor mode????

I just lost probably my 5th or 6th honor mode run to the myrkul fight. I’ve only managed to get past it once, it has killed every single one of my other runs. I’m so stuck?? Everything up to there is smooth sailing but I just can’t get past that fight. I even ended up trying barrelmancy in my last run, but my 10ish smokepowder barrels + runepowder bomb probably only did about 100 damage combined. I have no idea how to get past this and i don’t know how i did it that one time. Help???!!