This is not very difficult and can be done in 2-4 co-op games.
You have to play as ENGINEER and use the shield restoration on other people to restore their shield. Hit (LB) to pull out your "shield restoration pack" and restore with (RT).
I don't play Engineer usually so I went just for this achievement. Choose a normal or hard map. On easy people don't lose much health and checkpoints are too frequent.
Just stick behind and restore whenever possible. I got this during my 3rd match.
This achievement is cummulative over multiple matches and you can check your progress unter the challenges tab.
From the main menu select PROGRESS, RB to CHALLENGES, left column MARKS, scroll down to ARMOUR PLATE. It says xx / 10000.