My brother and I obtained this achievement after being stuck on 28/29 for a while. We tried training Carmine at the start but it still didn't unlock. After starting a new co-op campaign on casual, we both got the achievement during the end of Act 4 Chapter 3 when the game was saving a checkpoint. The advice I would give to people having trouble unlocking this achievement is the following:
-Do the training mission.
-During a co-op session if you quit and continue at another time by loading from a checkpoint, that particular chapter WILL NOT count towards the achievement, you have to start that particular chapter from the start again (Had to find out the hard way by playing the campaign again until the achievement popped up as we did not know what chapter we didn't do fully).
-If you die you get the 'Objective Failed' screen. Here you can load from checkpoint and continue the campaign as long as you don't quit the game (i.e. to the menu, lobby or dashboard) and that chapter will still count (i.e. doing the whole campaign in one go).
-You do not have to watch the cutscenes fully as some people have been saying.