I wanted to say Selen too at first, but I normally beat her in normal melee combat and a few magic hit when the barrier go away. Probably consider her more since my buddy didn't do well against her and we had to copy blind sword for him to have the trophy. But then, AnCapaillMor mentionned satsuki and that reminded me of my first playthrough. I didn't understand the world tendency at the time and was starting my NG+. I got invaded a bit too much in 4-1 and die a lot. But with the stack of Neph eye I revived and kept going. Until I left for the nexus and came back. I've never die or try so hard in my life to get pass him. I could probably do better against him now, but back then, he was my worst nightmare.
But that's it tho, the BP in DarkS don't stand near those from demonsoul. Because in DeS, those BP had way higher HP and dmg then the human form. Human form is what we get for BP invader in DaS, so that's why it's so easy. It would have been harder if there was some kind of Gravelord version of them, that would have been Swwweeett!!