Unlock the following Elite armor permutations by fulfilling the requirement listed:

Ascetic (Body) - Get the Up Close and Personal Achievement
Ascetic (Head) - Get the Steppin' Razor Achievement
Ascetic (Shoulders) - Get the Overkill Achievement
Commando (Body) - Get the Triple Kill Achievement
Commando (Head) - Get the Overkill Achievement
Commando (Shoulders) - Get the Killing Frenzy Achivement
Flight (Body) - Complete Tsavo Highway (on Heroic or Legendary)
Flight (Head) - Complete the campaign mode (on Legendary or Heroic)
Flight (Shoulders) - Complete The Ark (on Heroic or Legendary)