Those Left Behind Achievement in Halo: The Master Chief Collection

Towards the end of this mission, you will stumble upon a large city street section where 2-3 Marines are taking heavy fire from Covenant forces, including pesky Jackal Snipers. For the purposes of this achievement, you’ll need to protect these Marines and complete the rest of the mission with at least one of them still alive. 

If you’re playing on the lowest difficulty setting and with at least one other player, this can be done with minimal effort. When you reach the Marines, allow one player to hang back and engage the Covenant forces from afar with a decent medium-ranged weapon. This player should keep an eye on the Marines to ensure at least one is still active. All while this is taking place, the second player should rush down the street and past the two Hunters that emerge from a locked gate. Continue to run past these beasts, through the gate, and onto the next street until you reach a large amphitheater-type area. Drop into the area, exterminate the 3-4 Grunts, and then interact with the ODST drop pod to finish the mission. The achievement will unlock so long as that first player kept one Marine alive.