This is essentially the same achievement as
so read the solution there. They will unlock at almost the same time.
For the 'secret' ubercommander #16 in Riverside bunker you don't have to kill the boss next to him. Just melee the ubercommander (in fact melee all of them in the game), pick up death card and quit/return back to ship.
The other addition I would add is that if you didn't amass enough enigma codes by the end of the game, you can farm them at the Penthouse District in Manhattan. You will get 2 easy ones in about 30 seconds. Its a large room with a cellar. One guy is below you next to the cage and the other in the cellar. Rinse, repeat until you have enough.