Armor is a type of protection, characterized by a yellow bar on top of the enemy’s health bar. To get the achievement, you, or a party member, must use the Incinerate power to deal the finishing blow to an enemy’s armor bar. Krogans, YMIR mechs, Harbingers, and Scions are the enemies that most commonly have armor. Blood Pack Vorcha warriors will also occasionally have them, and a good way to boost the achievement up, as their armor is a lot weaker in proportion to the other enemies. I found that Mordin’s loyalty mission was a great place to earn this achievement due to the high amount of krogan and Blood Pack members you will face.
Note: Find a group of enemies with armor and destroy them all using incinerate. Then reload your checkpoint. It will keep your progress and you can repeat the process until you get the achievement.