Okay, first of all, I know, there's no such thing as official Mythic difficulty in Halo 3. That's what people call playing solo, legendary, all skulls on (HSH calls them SLASO to create more acronyms).

Daniel "Tyrant" Morris has posted over at HBO a walkthrough for this mode that makes this possible, and it's fantastic. If you haven't read it and you're a campaign junkie, it's an absolute must.

UPDATE: Author corrected from Asmodeus to Daniel "Tyrant" Morris.