Catch 'Em Napping
From Halopedia, the Halo wiki
Catch 'Em Napping is an achievement in Halo: The Master Chief Collection. This achievement is obtained by killing five sleeping Grunts on the level Mombasa Streets. To earn this, the player can kill any sleeping Grunts through any methods as long as the player did not wake them up. On some occasions, a Brute may guard a group of sleeping grunts.