Hold Y and then press X (times) and then press B (once).
In order to unlock the Authentic difficulty, clear the game on "Hard" difficulty. This difficulty has no aiming reticules, suppresion icons, and you cannot heal or revive your squad members by dying three times.
If you are having trouble on the Hard Setting, go to the in-game menu and press X, Select Unlimited Ammo and continue. Its not cheating because you unlocked it. No if you have grenades use them. Life will be a whole lot easier
If you are killed three times in a row, the game will offer you a chance to heal any injuries your squad might have sustained during combat. To expedite this process, you can just throw grenades at your feet.
Create a new profile and name it BAKERSDOZEN and all the levels will be unlocked.
Authentic Mode - Beat "Brothers in Arms" on Difficult
Unlimited Ammo - Beat the game on Normal
Old Movie - Beat the game on Easy or Normal
Super Squad - Complete game on Authentic mode