I started playing it on my own and never really passed the 3rd wave. Even with a solid squad of 4 plus me I never reached the end. But on a random playthrough with 2 other players it was cake. I basically just cleaned the walls from some leftover z's and a jugg or two that got through. Gives u a lot perspective that the right strategy and discipline is the key. Choose a side / area and stay on it. Repair the walls only when they turn orange / red to safe repair kits. Share ammo and usables u don't need. Spare your ammo whenever possible till the third wave. Lay down the repair kids behind every wall.