Campaign Complete: Heroic is a Halo 3 Campaign Achievement. It is obtained when the player completes the Campaign on Heroic or Difficulty or higher. This Achievement gives 125 Gamerpoints to the player and unlocks the Campaign Complete: Normal achievement if not yet earned. It can be earned through either co-op, single player or a combination of the two.
One way to achieve this is to complete campaign on normal, which will allow you to grow comfortable with the button layout and the new systems. Tactics become a necessity, as well as weapon familiarity. While it is not as difficult as Legendary, it is certainly more difficult than Normal. Do not get discouraged, perseverance and dogged determination will serve you well in completing the campaign on Heroic.