The Mythic II Map Pack Add-on for Halo 3 has 7 achievements worth 175 gamerscore.
On Citadel, get a Shotgun Spree during any ranked or social match.
On Heretic, get a Sword Spree during any ranked or social match.
On Longshore, get 3 kills with a Ghost during any ranked or social match.
On Citadel, find the hidden skull.
On Heretic, find the hidden skull.
On Longshore, find the hidden skull.
Find all the hidden skulls on the Mythic maps.