For this Achievement you need to keep the Elephant from taking any Damage in Operation A, Mission 3. When the Mission starts you can progress all the way to the end of the level before starting to escort the Elephant and killing every enemy along the way.
You can also complete every Assault Op on this Mission while clearing the level. Use your Assault Rifle to kill all Grunts and Shotgun every Elite and Drone so you will get them all in one go.
Then go back to the beginning and board the Warthog Turret, keep an eye on your Motion Tracker for any spawning enemy and keep shooting at their direction with the Turret. You can also stop the elephant from moving any time by ejecting from the turret if you wish to kill the enemies by foot.