No One Left Behind

No One Left Behind is an achievement in Halo 4 and Halo: The Master Chief Collection. It is awarded for saving at least one Marine in Chapter 3 of Episode 2 in Spartan Ops on Heroic or harder. In Halo 4, the achievement is worth 20 Gamerscore and is represented by the number "2" inside a circle. In Halo: The Master Chief Collection, it is worth 10 Gamerscore and is represented on the Steam icon by an enclosed Marine CH252 helmet in front of a wreath and 3 stars.

Tips
There are three pairs of marines at three locations around the map. From the start of the level, these six marines are invincible. Once the player kills the Covenant besieging a pair, the Marines will join the player. Once the player retrieves all three pairs, the marines will become vulnerable. Once this happens, the player should immediately proceed up the main structure; the marines will (or at least should) follow the player up the ramp. To minimize the chance of marines being left behind, be sure to rescue the marines furthest from the ramp first.

At the top of the structure the player will find numerous regenerating caches of M41 rocket launchers and M6 Spartan lasers. Most importantly, this location is far easier to defend, as it forces the subsequent waves of Covenant reinforcements to funnel up the structure's single central ramp. Once the last Covenant attacker has been killed, the achievement will unlock.

It's recommended to arm all of the Marines with heavy ordinances (such as Spartan Lasers or Rocket Launchers), as they can help the players destroy Phantoms when they defend the main structure. The Elites, especially ones wielding Energy Swords, often ignore the player, and instead focusing on attacking the Marines. As such, the players should defend the main ramp and ensure none of the Elite reach the Marines.

Trivia
The achievement was originally announced with the name "Rescue Ranger" on Halo Waypoint. This name may have been a reference to the 1980s children's cartoon Chip 'n Dale Rescue Rangers. By the time the game was released, the name was changed to "No One Left Behind". However, the Halo 4 stats page in Halo Waypoint still refers the achievement as "Rescue Ranger".