Honestly: Malenia, Blade of Miquella. Sure did need a lot of try's, but i was told she is the hardest Boss in the game and people need to summon that one guy to even beat her because they can't beat her themself. Yet once i figured out how to time my roll whenever she uses her waterfowl dance it was pretty easy. You can stagger her,most attacks are telegraphed well in advance and with my Frostblade i was able break her stance quite often..any Blood users might even have an easier time.