You will need 4 good players total for this probably. (Most efficient) and play Easy.
Play any campaign, one that you all want to practice.
I believe Blood Harvest to be easiest.
Rush, and move swiftly, whilst staying close together, especially when Hunters, Smokers, and Tanks are nearby.
Spam melee when Hunters and Smokers are nearby to hit the hunter or your ally to release them quick enough to avoid damage.
Smokers grabbing you, will not do damage until he is next to you or traps you against an object.
Hunters don't do damage until a little bit after they pounce you.
Boomers can only damage you by melee which doesn't happen often. NORMAL INFECTED DAMAGE AFTER BEING VOMITED/EXPLODED ONTO DOES NOT COUNT. That is commonly not known...
Tanks damage you by melee or thrown objects. Kill them by everyone shooting him quickly. Molotovs help. Make sure to use pipebombs and molotovs to keep infected away or else they will flank you and trap you so the tank will hit you.
Witches can be avoided or killed with shotgun, your parties choice. When they run past someone and stun them, it does not count. Apparently witch damage doesn't count. I'm not sure if this was always this way or in an update. Let us know if it works for you even when getting hit by a witch.
UPDATE: This can also be done on the DLC map Crash Course. It is much easier since there are only 2 chapters.