Once you reach level 11, go to the southeast corner of the map. Waves of 20 enemy soldiers will try to get to the entrances of the compound if you let no more than 20 get in you will unlock this achievement.
When going through this game which I am highly recommending doing is using the Xbox Achievements road map made by CienPR, when you hit each level of the story just hit CTRL+F on the page and type the Mission you are on, and you will find the achievements you need to look out for and how to do them on each mission. There is about 2 each mission, and there aren't any videos which I would be posting if I could find some, but I guess this game isn't popular enough.
Again credit goes to CienPR, good luck and have fun