Once you get to the armory in level 5, save the soldiers and the achievement will be yours. Achievement will unlock after the medal is awarded.
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