Shields are heavily armored units carrying ballistic shields, making them impervious to frontal attacks. To defeat them, you must flank them, shoot their exposed back, or use explosives. Shotguns with 'AP Slugs' or 'HE Rounds' can penetrate their shields. Alternatively, use a melee attack to stun them and then shoot their head. Coordinate with your team to draw their attention and expose their weak points.
