It varies between the players, but Pthumerian Descendant is harder than anything else in the chalices for me. I've never managed to beat him solo. If you beat Abhorrent Beast in hand-to-hand combat, instead of cheesing with poison knives and Bone Marrow Ash, you might not have such hard time as I did.
After like 60 tries
The one that I could not beat solo was Laurence, I just couldnt do it.