Simple stuff really, just use 2 controllers, let off a round so it properly registers 0% accuracy, and then put that controller down, go ahead and finish the level with the other controller.
My experience: I was actually finishing the campaign in coop, my coop buddy had to leave so I soldiered on alone. just putting the controller down. They got 0% accuracy and I got something like 70%, but I still got the trophy.
Feel free to comment any suggestions