If you'll forgive a bit of bragging:
The Orphan of Kos is probably the hardest video game boss I've ever fought (though to be fair, it was Dark Souls' Ornstein and Smough who prevented me from progressing further in that game.) The massive health pool, devastating and hard to dodge attacks, and relentless pace of the fight all combine to make this a huge challenge.
On my first character, it took me at least twenty, maybe thirty attempts to take him down.
On my Arcane character (who is technically my third but has overtaken the Skill one by one boss) I took him down on my first attempt.
Now to be fair, I've probably leveled up more on this character, though I don't think my a huge amount (the Strength character was at like 45 strength while the Arcane one was at 50 arcane, though I'll also note that I have some underwhelming fire gems.)
Still, while the fight left me with zero Blood Vials, I managed to vanquish this foe in a single attempt.
This actually makes me question a bit whether my initial assessment of Orphan of Kos as the hardest boss in the game (or potentially gaming) was accurate. On the same character, Laurence, the First Vicar, took me several attempts (like the Orphan, I beat him with zero blood vials left.)
Anyway, difficulty aside, at some point I want to lay out my theories about the Orphan of Kos, Maria, and the reason the Hunter's Nightmare exists. I haven't totally nailed down what I believe, but I suspect that the death of Kos is a kind of "original sin" of the Hunters, and that perhaps the entire Curse of the Beast may be the retribution of those in the Fishing Hamlet.
So this also leaves me at a kind of "clean-up" stage with my Arcane character. I think I'm going to wind up doing the three cords ending again, as I want to see how well the Burial Blade works for Arcane. Still need to get the Rakuyo, a couple of the Hunter's Tools, and I think the Gravewarden set in the Forbidden Woods (which I imagine I'll be able to tear through.)
I really want to like the Kos Parasite weapon, given that I'm on an Arcane character, but perhaps given its lack of appropriate gems (it's a default Arcane weapon, meaning elemental gems of other types can't fit it) I'm finding it sadly weak.