To unlock this Achievement, you must complete an entire Chapter without being hit a single time.
This appears to be easiest on Chapter 2 - Trial, since you've got a pretty low number of enemies and they aren't too aggressive yet. Direct Lynch to take out all the threats while you lay low in cover. Keep your eyes peeled and take out anybody that gets by him.
Be wary of the final wave that comes out once Kane reaches the ground floor. Try to position Lynch somewhere that he can easily protect you before you venture downward.
Feel free to drop the game down to Aspirin difficulty level, as it does not affect the Achievement.