Easy "Dr. Lobotomy" Achievement Or Trophy:

Successfully complete the game, then start a new game in Headshot mode. Complete the first two scenes of the first level. Exit to the main menu after the third scene starts. Select "Unlocked Acts" and choose the final level. Select any character and choose the same difficulty setting played under earlier. Ignore the message and begin the level. Complete the entire last level. You can activate cheat codes, but must disable them during the final scene against Immortal.