Simply complete a level and don't get hit more than 30 times. Make sure to count how many times you are hit and if you think you've failed before the end, restart te level. This is done easiest on the first level at Saint Lo, just keep your head down and shoot occasionally to help your teammates (who will help out a lot) keep the enemy at bay. You can do this on any difficulty, but obviously its easier on Easy. You can also get the "Still Ticking" and "Conservationist" achievements on this level.