I'd expect most people to die to the Flamelurker and Penetrator. Old King Allant and the Maneater are other great candidates. I can see all of the others getting beaten on a first try even without "extensive experience with other Souls games".
If you expect people to die to four level bosses in Demon's Souls, that doesn't stand in contradiction to my post about beating 12 out of 16 level bosses on the first try.
Though I did defeat Penetrator (with assistance from Biorr of the Twin Fangs) and Flamelurker on my first try, and would have beaten Maneater if not for falling off that walkway.
I think Demon's Souls is fairly easy in terms of action. It's the dungeon design that's hard, and From has made dungeon design easier and boss design harder as the series has progressed.
For example, Demon's Souls lose 50% of their health and become souls form after dying in their Human Form.
If you consistently die in a particular dungeon (meaning Human Form), the difficulty of that dungeon will increase. There are ways to reduce the penalty, but if you don't realize this and keep dying, you'll end up in a weird situation where the difficulty keeps increasing.
You might be confusing Demon's Souls with Dark Souls II. The former starts you off with a 50% health penalty in Level 1-1, after having died in the tutorial and arisen in the Nexus, but you can quickly obtain the Cling Ring that halves the health penalty from being in Soul Form, and in any case this penalty does not increase from further deaths. Dark Souls II, however, has a small 5% penalty from dying that is applied cumulatively with each death, up to a maximum of 50%, but health is fully restored any time you restore your humanity, and the health penalty from later deaths then restarts at 5% from your maximum health.