Because the servers are closed for this game, you will need to play local co-op to gain this trophy and thus you will need to have two controllers.
To get this fast (and/or solo), the easier mission to get this done on is Mission 4, "Last Resort", which only has two, rather short chapters on it. For an even easier experience, choose the "Casual" difficulty and you can stand out in the open with barely a care in the world as you eliminate the enemy AI. You can do this during the "Mission Select" screen once you have played this mission during regular play. The rest of this tip assumes that you are doing this solo.
Mission 4 is the mission where you meet up with two of your TWO partners at a hotel. Each chapter here is short and the enemies on Casual difficulty are rather small and fall with one or two shots. There are barely any hard spots and the mission can be completed on your own, controlling both Alpha and Bravo, within 10 or 15 minutes. Even if you have some difficulty with it, ie you haven't upgraded any weapons, you can use the other character/controller to revive the one you were playing on. To be safe, make sure that the controller you aren't controlling has the player behind cover by using
Even those that aren't very coordinated when it comes to using two controllers at the same time should earn this trophy rather easily, particularly when the difficulty level is set to it's easiest setting.