Instructions
Obtain a 100 killstreak during the campaign or during Firefight mode. This means you cannot die in between any of the 100 deaths. You can either set the campaign or Firefight mode to Rookie difficulty or you can set invincibility in Firefight. This unlocks Jun's helmet.
Follow one of your teammates during a multiplayer match. Defeat an enemy as soon as one defeats your teammate. If timed properly, this gives you a special "avenged teammate" experience bonus along with unlocking Kat's helmet.
Play either Firefight or regular multiplayer with a powerful, short-range weapon. This can be a sniper rifle or a shotgun, though the latter does not require much aiming. Defeat any enemy that begins swinging the energy sword in mid-dash, earning you Emile's helmet.
Complete any mission in the campaign on Legendary difficulty. You can have friends join your game and assist you or do it alone. Because each mission increases in difficulty and the number of enemies, the first mission is the one where you can accomplish this with ease. Completing the mission earns you Carter's helmet.
Defeat five players in a row within four seconds of each other in multiplayer. This means the first and fifth players you defeat must not have fallen more than four seconds of one another. This will require the most practice, but completing it earns you Jorge's helmet.