If like me you have two controllers but no one to actually play with, this is still easily obtainable in Act 1 on casual difficulty. Although it's not the shortest act, it is the most straightforward without any vehicle sections that mean there is nowhere to take cover.
For the most part, the second player can be just left idle behind a wall or around a corner safe from harm, although there are two spots which might require a bit of dexterity on your part. The first is in the prologue where you have to run away from the mortars, but the path is pretty much a straight line so holding
simultaneously should get you to safety. I've never actually been hit during this part so it might even be scripted for you to avoid damage. The second is where you're returning with the fabricator on the rising platform, since there are no truly safe spots. So try and raise one of the side barriers with your dummy then hide behind it, getting ready to switch controllers in a hurry should one of you go down.
For the final horde section, player 2 can just be left behind one of the pillars of the main door. After the cutscene where the Guardian blows up the statue they will teleport forwards however, so be ready to move them. Achievement will pop as soon as the final cutscene starts.