This can be done on the very first mission on Act 1 were you have to go to the slaughterhouse to release Vittorie from execution. Take the sentrys uniform by stealth killing him and just hold lb whilst moving to the objective, this helps keep suspicion down. Also if your circular bar in the lower left corner turns yellow, move quickly away from guards who are near, it will go white to show your safe. If the bar goes red the alarm will be raised. Once near the cell that Vittorie is imprisoned, stealth kill the guard outside the cell and take the cell keys. Once cut scene has passed run and follow the others to the downstairs basement before the british bombers blow up the slaughterhouse. You don't need to go through the basement doors as the mission ends as soon as you are near it.
As long as the alarm hasn't been raised this will unlock the achievement.
PS To the person who voted negative....why? The above worked for me and may work for others. Thats why its a guide muppet! Jeez some people are complete morons. Why not PM me first to point out any errors? Maybe he/she is thinks they are real clever, well think otherwise!