Mark of the Ninja

This is doable on the second level, but it's worth noting that the next-to-last level (Set to Flight) is a great place to get this as well - I found the fastest way to get the points required for 3 stars in that level was to avoid killing anyone and collect the bonus.

Also, late in the game you'll be awarded a skill called "Serenity" that will make this far easier, so don't be afraid to save this for New Game +, where you'll have that skill from the beginning.

You'll get this automatically when you unlock "Ghost" for completing a level without killing guards and without being detected.

EDIT: With the advent of the new DLC, this is even easier. The costume you unlock by playing the DLC level changes your stealth kill into a stealth knockout, so it's trivial to go through a level with that costume equipped ("Path of Wisdom") and kill nobody.