Equip your shield and an assault rifle, shot gun, rifle, bazooka or placeable weapons (no handguns, sub machine guns or grenades). While firing, turn your back toward the enemy as they open fire. The shield remains on your back, deflecting shots coming at you. This works well when you are overwhelmed or when you lack means of cover. This technique doesn't work well against heavy machine gun fire, snipers, heavy soldiers or any soldier with a bazooka. Also, since you don't have rations equipped, you cannot heal. Get some experience with this technique with the russian invasion level.