I got this by simply loading up split-screen co-op using a second controller and playing five levels on easy difficulty using one of the controllers. Just let the other controller sit next to you, leaving the second player standing at the beginning of the level until you need him to advance to the next area.
The only difference between single player and playing split-screen co-op using a second controller is that you need to move both players to certain gates, doors, and walls at the same time in order to advance to the next area.
Clear all enemies in each area using one controller and when you reach a gate, wall, or door that needs both players, move your first player to the gate, wall, or door, pick up the second controller, and run that player all the way to the gate, wall, or door as well. Hit X on both controllers at the same time, and you'll advance past the obstacle.
If you play on easy difficulty, you can rush the enemies without much trouble. The enemy AI isn't very adaptive when you rush them; they tend not to switch cover when they ought to, so you can often run up behind or alongside them, easily taking them out.
If you die with your main player while far away from player 2, don't waste time grabbing the second controller and trying to revive the downed player. Just let him bleed out and start again at the latest checkpoint.