Giant Slayer is currently bugged. Having done the requirement multiple times, it did not pop once. This is tied to the fact that the PS4 version of the game isn't tracking what your character does (why merchants, ancient hunts and other trophies are bugged). I am playing on PS5 and this is still the case there (as it uses the PS4 version of the game). The only workaround is to use your pet to get the kills as that does track.