Shields are a type of protection often used by mechanical enemies and engineers. You, or a squad member, must deal the finishing blow to the shield bar with the Overload power 25 times to earn the achievement. Blue Suns mercenaries have the most engineers with shields, and YMIR mechs also have shields. I found a good amount of Blue Suns engineers on Garrus’ loyalty mission.
Note: Find a group of enemies with shields and destroy them all using overload. Then reload your checkpoint. It will keep your progress and you can repeat the process until you get the achievement.