Just got the No One Left Behind achievement.
Kinda tricky-took a few tries to get it because the marines are dumber than a sack of shit.
How I got it was three-player co-op. My buddy ran with the energy sword and took out as many elites as possible, we grouped up on the hunters and the wraith, and I ran up top asap to take out the phantoms at the end.
100%: CoD2, Halo 3(1750), Halo 3:ODST, Dash Of Destruction, Red Dead Redemption(1500), Rock Band 2, Halo Reach (1700), Harm's Way, Doritos Crash Course, Alan Wake(1500), Portal 2, Halo CEA, Halo 4 (2000), Minecraft, 3 on 3 NHL® Arcade, Watch Dogs (1000), Halo: Spartan Assault (1000).
Soon: Halo MCC (6 left), Watch Dogs (DLC left), Far Cry 4 (DLC+2 left)