You have to finish a level using less than 300 rounds of ammunition.
Best level for this is the first level, at Saint Lo, as you can rely on your comrades a lot.
Tip: Use a Rifle, rather than an Automatic weapon. This means you should only use one bullet to get the kill, not 30.
P.s. Don't worry about dropping the difficulty down to recruit. Difficulty doesn't affect this achievment.