This can be tricky depending on what missions you attempt to do it on. The first one that I did was when you have to transport the Doctor in the tank. Make sure you don't fire AT ALL, and you won't be discovered. You should also try and avoid running over the military.
The second one would be the mission after this one to protect the doctor. Again, stay disguised the entire time.
Other fairly easy missions are when escorting the Bloodtox Pump; the one after that protecting the Bloodtox Pump; or an earlier mission when you have to escort the Thermobolic Tank.
Important: The stats for this carries over to a New Game + so if you can only get one or two of these done without alert, you can do the same exact mission again on a different playthrough to finish off the achievement.