Complete all 11 levels on Agent difficulty. If Agent difficulty is to hard, there is a trick. Play the level on Trainee difficulty until the last autosave checkpoint, exit to main menu, then chose "Continue Chapter". Change the difficulty to Agent for the last scene or boss fight.
Here is a good guide for when to exit out of each level (Thanks Younikorn):
http://www.xbox360achievements.org/forum/showthread.php?t=75...