SomeFilthyCasuals have recently released some tutorials, detailing how to fly and drive the Mammoth in the Halo 4 level, Reclaimer. This is done by using some load zone skips to de-load the Mammoth's AI, causing it to become movable by other vehicles, like the Warthog or Pelican. See the tutorials below for more details (there are text tutorials as well in the video descriptions):
Flying the Mammoth: https://www.youtube.com/watch?v=upX87FZou1s
Driving the Mammoth: https://www.youtube.com/watch?v=xbMwUGaqzpI
Thanks for watching!