Silent but Deadly Achievement in Halo: The Master Chief Collection

Contrary to the achievement's description here, killing seven enemies does not necessarily have to be from behind. Rather, so long as you "stealthily" kill seven enemies in a row without your position being revealed, you'll be fine. There's two distinct strategies in obtaining this achievement, so use whichever works best for you.

On one hand, you can load up Cairo Station on the Legendary difficulty setting and activate the Scarab skull in the Campaign lobby. When you reach the first hanger via the second floor, you will see some enemies on the ground level shooting at the Marines on the catwalk. Use whichever weapon you have at your disposal - like the Battle Rifle - and kill this first wave of Covenant. Then, be ready to blast the dropship's "tube" where the next set of enemies begin to spawn from. If you are playing in Remastered Mode, the corpses will get stuck in the geometry of the "tube" and remain there. At this point, the third wave of Covenant will begin to spawn, but their movement will be blocked due to the dead bodies. If you shoot your weapon at "tube" at this point, it will count as a stealth kill since the enemies are not technically loaded into the mission yet. The achievement should unlock here without issue, though it's not always guaranteed.

An alternate strategy is to load up Gravemind on Normal (or Easy). Here, you do not need to activate any skulls. When the mission begins, continue normally throughout the corridors of High Charity until you take an energy lift down to the prison cells. When you arrive, Cortana will inform you about two groups of Marines that need your help in breaking free from the Covenant's cells. After clearing out the middle floor and rescuing the first group of Marines, head down the bottom-most floor and approach the doorway that leads to the second room of prison cells. At this point, you should receive a checkpoint before entering the room. Make sure to wait around until this checkpoint registers. Once it does, enter the room and melee the Brute patrolling the center of the room; he will always have his back to you. After killing the Brute, go ahead and smack the sleeping Grunts in the room as well. Quickly revert your checkpoint once you kill the Grunts and then repeat this same exact process to get your final two-three kills and the achievement should unlock without any issues.