Velvet Assassin

This is easily done in the first level of the game where you will only have to worry about 5 guards. The first one is easily killed since he is drunk and will never turn around to see you. When you come across the bridge with 2 guards, wait until they separate. Quickly go to the guard still on the bridge, kill him, and dispose of his body in the bushes. Wait for the other guard to walk back to the bridge and start following him once he walks back to his post. Once you dispose of him crawl under the fence and hug the bushes and shadows to stay hidden until you get to the morphine packs. Try to stay hidden in the bushes until you get to the large rock directly facing the guard. Hit X to use the morphine and rush towards the guard, killing him while still in the morphine state. Search his body to get the key, unlock the door, and the level will end, netting you the achievement. As long as you don't ever get spotted by the enemy (especially the guard on the ledge near the level's end) you'll be fine.

You can play through this game the normal way...

Or...

You can use cheat codes. Plug in a USB keyboard to the 360 and type them in during gameplay. Achievements are not disabled when using the following codes.

Complete Current Level - Hold Left SHIFT + L, then press END

Yep, that's right. Start a level, use the code, finish the level. In this case, you will obviously have finished the mission without being identified, so... POP!

You can also get the achievement for the level, and work your way through for the other mission/campaign achievements. Should take you about 15-20 minutes to complete the campaign.

If you want to run through the game to get the collectibles, etc., you can also use the God Mode cheat code: Hold Left SHIFT + Right SHIFT + P, then press G.

I didn't come up with these... all credit to FR3SH JIV3 at www.xbox360achievements.org.