First, go to the farmhouse finale, go through the level like you normally would.

Now, when you fight the horde, when there is almost no sound, quickly pause the game, change the difficulty to expert, incap your teammates, change difficulty back to easy.

Do this upstairs in the hallway, leave your teammates there, and if done correctly, the tank should climb up the side of the house, and get stuck.

He will throw concrete blocks at you, so hide behind the wall on the roof - make sure you change the difficulty back to easy. Enjoy!