Ok so i just tackle this challenge today and even tho I died like 10 times, Its mostly a patience test, At least for me it worked to just move around and wait for an attack to end so i can reach its head, if he does a move where i knew my weapon could not reach it I went for the legs (My best weapon at that moment was the Beast Cutter so it was hard to really learn his moveset to know when he lowers his head enough for me to reach it). For what i can tell the runes that helps you max out hp are not worth it for this chalice, so i took runes to upgrade my visceral attack, to hold on more blood vials and to level up my arcane defense.