So here's my tips
No Mercy
Straght forward group up and watch yours and your team's backs.
Listen for the tell tale breathing of each of the special infected or the witch's weeping.
When the tank arrives set-up a cross fire and it should be no problem.
In the hospital when waiting for the lift grab enough ammo then don't wait by the lift go down the other end of the corridor only one entrance and your combined firepower is easily enough. Loads safer.
On the roof just play as a team zombie horde, tank, horde, tank then go for the helipad and don't get creative. Pipe bombs are a must for me.
Dead Air
The finale is very easy if you all go to the plane behind the one your are refeuling the zombies have to travel a long way to get to you so are easily cut down before they reach you. You just pop back to the minigun to rearm every time the tank comes. Same formula horde, tank, horde tank go to the plane and don't get creative.
Death Toll
The finale is a little more interesting as the variables are greater as there is no "best spot" . Horde, tank, horde, tank as ever so just stay grouped and don't let the smokers and hunters prey on lone fighters.
Bood Harvest is pretty much a set up of cross fire up the stairs in any of the rooms and just be sensible with the tank phases.
Versus is a totally different kettle of fish.