Firefight: Lost Platoon is an achievement in Halo 3: ODST. It is awarded for scoring over 200,000 points during a match of Firefight on the map Lost Platoon. It is worth 10 Gamerpoints.[1]
Trivia
The symbol for the achievement shows the New Mombasa Orbital Elevator, which probably means the level is situated in or around the structure.
Sources