Catch 'Em Napping Achievement in Halo: The Master Chief Collection

After competing ONI Alpha Site (through a mission select via the Campaign lobby), you will return to the Mombasa Streets and like always, be required to explore the city in order to find the next clue. When you regain control of your character, head over to the left side of the street, through the gate that you originally came from when finding ONI Alpha Site’s mission clue. Past the gate, you will encounter 5-6 sleeping Grunts and a Brute (they won't spawn here outside of mission select because during a natural playthrough, you would have already killed them coming to ONI Alpha Site's clue). 

The first two Grunts are separated from the group and can be smacked for a one-hit kill. As you walk up the street, you’ll be able to smack another sleeping Grunt, but this tends to alert the Brute and wake up the remaining Grunts in the area. Since you’ll already have three kills towards this achievement by now, simply revert your checkpoint and repeat the same method and the achievement will unlock once the fifth sleeping Grunt has been killed via a melee attack.