At the end of Mission 6: Special Missions HQ, you will encounter an enemy with a shield. A short cutscene will begin playing when you enter the room. Eliminate the other enemies, and let the Infantryman get close to you. Then, shoot his shield until he breaks down. You will then have a couple of seconds to get behind him and eliminate him with hand-to-hand combat. Pick up his shield to get the "Hostile Shield Secured" achievement.