The one you're referring to is a ridiculously tough. I think I got him down to half way over a few attempts, but I won't go back there for a while! Found parrying to be the most effective strategy, but when you get it wrong he punishes you HARD.
For me he was much tougher that the first two "main" bosses...finally managed to down him but it took many, many deaths
Remember this when you face him again: roll into his attacks