The easiest way to do this is to play on Easy or Normal difficulty.
Activate the skulls Catch, Rebel Sympathiser and Sugar Cookies and ram every Grunt you see.
It is a good idea to pull your warthog out of combat until the ram ability recharges so that grunt units aren't killed by the warthog turret.
Also use the warthogs found later in the level instead of Forge's Gauss Warthog as they won't kill grunts with the turret as easily.
- The title of this achievement quotes Marty O'Donnell from the Halo 3 Legendary Edition cut scenes with developer commentary.