See topic.
I have all the other requirements - I've beaten all the five-star quests; and now because I only have one Rath Talon, I cannot finish the "One Chance" request (which calls for two Gigginox Hides - I have 9, eight Machalite Ores - I have 6, but can easily get more, and TWO Rath Talons - I have one.)
I've fought Rathian again and again, focusing on her claws, but to no avail.