This should come along with normal play. Your armor is whatever Battleset you have on. On the top right of the screen is your health (a green bar). When you have a Battleset on, it shows an armor type design on it. As you take damage, it starts to chip away. You must have some left by the time the mission ends.
Easy Method: You can also gain these in bonus missions. Doing a Speed Flying objective for example could grant you this achievement very easily since the only way you can lose armor is to accidently fly into the environment.