Here i'm gonna show you some tatics for perfect playing... At first, i have divided some classes based on weapons you can use:


Striker
Weapon:Pump Shotgun/Auto Shotgun.
Actions:Front line of the team, this one is the tank crusher/Special infected killer.
Suggestion:Have two on the team.


Healer
Wepon: Hunting Rifle/Pistols/SMG
Actions:Stay behind the strikers, this one will provide pain pills for the strikers and also heal them. Using the hunting rifle will save the team from smokers and boomers.


Suporter
Wepon: Rifle(M4A1), SMG
Actions:Will protect the whole team from behind the healer. This class is a "noble" one, once heard the tank, he will go ahead alone to lure the tank for the Strikers.


Talk to your team and try these classes. (Every class carries an responsability)