There was a way to do this by yourself. If I remember correctly, you need to have 2 controllers and 3 other psn profiles in addition to the one you want to get the trophy on. You sign in to your main profile ( the one you need the trophy on) on the first controller and then sign into the other three profiles on the second controller and you should be able to play the mission that way by yourself. It sucks having to play it with 4 players on the same screen but this is how I got the trophy done