The Saboteur

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!

i found an easier level as i'm very trigger happy and can't always raise an alarm on "A Trip Up the Coast" in Act 1 it consists of only driving to a point on the map so as long as you drive safe you will be fine, I even stopped to blow up a few Towers and unlocked the achievement

Extremely easy to do on the mission "30 pieces of lead" which is just a bit after the start of the actual game (first mission from Vittorio after the prologue). All you have to do is to follow a Nazi general without being detected, and killing the traitor in the end. I followed him easy and gunned them both down for good measure, zing, achievement.

An even easier mission to get it on is Act 1 A Trip Up the Coast.All you do is drive the route on your map that is in yellow and don`t hit any nazis that will be on the sides of the road sometimes.

This is one hell of an easy achievement. It asks you to complete a mission without raising the alarm.

This is best done on Act 1, Slaughterhouse.

Once you receive your mission from Luc and Veronique, head outside and stow your weapon before the Nazi soldier gets even more suspiscious. Find a car and drive to the location that's on your GPS.

Once there, Luc will explain the mission further to you. Make your way to the other side and climb up the steps, making sure that the Nazi soldier does not spot you. Once he turns around and walks away, hold down LB to sneak and climb up the steps, making sure to get behind him.Once you are close enough from behind him, make sure to perform a stealth kill by holding LB and hitting either X or Y, then grab his gun and take the uniform, disgusing yourself with the new outfit. When that's complete, make your way to the tunnel and begin the infiltration.

You'll deal with a lone guard, so perform another stealth kill and make your way into the base. There will be a ton of Nazi soldiers, so just hold down LB while walking and you should avoid their suspision, even if you are near them. If they get too suspiscious, walk away for a bit and let the meter cool off. You will need to find your way inside the cell prison, which isn't hard to find the entrance for.

Once inside, make your way to the lone guard that's near the prison cell. When his back is turned on you, go ahead and perform a stealth kill on him, then grab his keys and open up the door. Vittore and Luc will join you soon after and you need to run towards the basement. Follow your GPS and make your way downstairs. Once you reach the door, a cutscene of the British bombing the place will ensue. Once that's complete, the achievement should be unlocked right then and there. Very simple stuff.

Have fun and good luck!