Death from Above
From Halopedia, the Halo wiki
While possible on any difficulty, it is best done on Easy or Normal. After encountering the second Shrike turret, head towards the left into the room that has a circular grate on the bottom floor and a staircase to the right of it. The room should be filled with a few Grunts.
Climb up the staircase and wait until a Grunt gets very close to the grate, then jump and do a Ground Pound on the grate. If the grate breaks and the Grunt dies, the achievement is then unlocked. However, watch for any Jackals nearby and kill them in order to avoid being downed.