The achievement “Who Needs Them?” does not unlock in the PC version of the game. This seems to been an issue since 2015 according to this thread (https://www.halowaypoint.com/en-us/forums/85d779d52cfd46918b4f8b638f2e3c7b/topics/who-needs-them-achievement-glitched/4838d771-bf4c-4ea1-92ec-a46bdc9caa9f/posts?page=3)
Who Needs Them?
Halo 3: On The Ark, complete the level without entering a vehicle on Legendary.