This achievement is obtained by completing all levels in the game (Halo CE, Halo 2, Halo 3 & Halo 4) on easy difficultly.
Mission difficulty must be at least easy, however, you can complete them all (or some) on normal, heroic or legendary - as the achievement stacks.
You must not use any skulls that void achievements. These skills have a warning on them before you start the mission and they have a x0.00 multiplier.
If you are having issues with the achievement unlocking, give the game a restart or (unfortunately) you may have a wait a week or so - according to forums. One other solution I have noticed is going to the game hub and heading over to the progess tab, this sometimes unlocks stuck achievements.