This is how I got this achievement, "Got Your Back". This solution will only work with two controllers.
First finish the campaign by yourself, then "Replay Chapter" #1 and make it a splitscreen match. Play Chapter 1 (only one mission and it's really easy), then play the first mission of chapter 2.
After you finish chapter 2 mission 1, go back to the main menu, select Chapter V (5) and play all the missions. Using this technique, you will get all 3 co-op achievements easily.
NOTE: If you do not have two controllers, find a friend to play co-op with you, or hope you can find a match online.