To unlock this, what you need to do is play and complete a match WITHOUT "Auto Armor" selected as a Suit Module.
Go to> Customize Equipment> On the Left there are (3) Module selections. Just make sure you don't have "Auto Armor" in that class, and play a match with that class.
During the match, press "LB" to manually select Maximum Armor. You don't have to do this step, but if you want to survive a spray of bullets, you might want to press it :)
An Easy 10 Gamerscore.
Edit: Turns out you need to activate armor during a match. So, press "LB" people.