Black phantom mindflayers weren't that bad; baiting the paralyzing attack gave you free hits and the spiraling stairwell made it hard for them to get a direct angle on you.Not talking strictly in terms of platinum, but overall, I don't think Demon's is too hard all around, but I would say that I had some of the more... unforgiving and rough moments out of the entire series.
I can't speak for the Defiled Chalice dungeons in this game, but even the brain woman bridge didn't give me anywhere close to the same amount of dead as fighting a Black Phantom Mind Flayer on a narrow-as-fuck stairway.
I'd actually say the hardest part of Demon's Souls main game was 4-2, since there were so many enemy ambushes that could take out a healthy chunk of your life bar.
The brains in Bloodborne... Argh. They can go fuck themselves.