From Halopedia, the Halo wiki
Assault is an achievement for Halo 3 and Halo: The Master Chief Collection. In the original game, it is worth 30 gamerpoints and is unlocked by beating The Storm on Normal, Heroic, or Legendary. In The Master Chief Collection it is only worth 10 gamerpoints, though it can also be unlocked on Easy.
In the original game it is represented by a blue plate with a roman numeral IV on it. In The Master Chief Collection it is represented by a screenshot of the level.
Note: The player must finish the level completely, beginning from Mission Start, or they will not get the achievement.