Enigma Codes are items you can collect from dead commanders (targets that appear in stealth sections of the game). To unlock Ubercommander assassination missions, you must collect and use them in the Enigma Machine. To quickly get Enigma Codes, select the "Manhattan, Penthouse District" assassination mission at the war map (unlock it if it is not already unlocked). Set the difficulty to the lowest setting under the "Gameplay" option at the options menu. Successfully complete the mission once. Each time you play the mission afterwards, you will find two commanders, allowing you to get two Enigma Codes each time you do this. On the lowest difficulty, you do not have to worry about damage. Drop down and kill the first commander, then drop into the ruined area below and kill the second one. After looting the second commander, open the map and exit it. Ignore the enemies, as you do not need to kill anyone else. Just sprint straight to the commanders, kill them, loot them, and exit. Repeat this as many times as desired to get an unlimited amount of Enigma Codes. This method is also great for maxing out all your perks to reach 100% completion. Lower the difficulty, complete some stealth or combat kills, then exit and repeat. You can raise the difficulty later with no penalty.