There are 9 main missions to this game and difficulty DOES stack. (meaning you can earn this by playing on easy, normal, heroic, OR legendary)
Just beat the follwoing nine missions and watch the games starting/ending cutscenes.
Mission 1: Sierra 117
Mission 2: Crow's Nest
Mission 3: Tsavo Highway
Mission 4: The Storm
Mission 5: Floodgate
Mission 6: The Ark
Mission 7: Covenant
Mission 8: Cortana
Mission 9: Halo