Mark of the Ninja

Like "Mercy" - complete a level without killing a guard - this is probably easiest on the second level. You can restart any checkpoint when you're detected (red alert only - guards can get the yellow icon over their head and you're fine).

It's also doable on the 11th level, "Set to Flight". I found that going through that level without killing anyone and without being detected was the fastest way to get all of the points needed for three stars. So if you have trouble on level 2, just wait and try that.

If you're still having issues, know that a skill you're awarded late in the game ("Serenity") will make this much easier, and you can use it from the beginning of the game on your New Game + playthrough. In fact, you can even go back using level select and try it on level 2 after you have the skill. It should be a cakewalk then.

EDIT: The new DLC introduces a costume that gives you another easy way to get this achievement if you don't like Serenity for some reason. The Path of Wisdom costume changes your stealth kills to knockouts, so you can just knock out everyone in the level as you go. The first level is a good place for it. Just make sure you equip non-lethal equipment.

You must complete the level without killing any enemies and being detected. Achievement voids if you do the following:

1. you get detected by enemies

2. you kill any enemies.

Should this happen to any players, restart the level to try again. Achievement pops if successful.


Note to self, upon popping the ghost achievement, you'll pop two other achievements.