"This is going to hell in a hand basket real fast!" "Axe me a question!"
Oh boy, do I have good memories of L4D2, and surprised by the fact there was no option to disable team kill, not even in "as if" scenarios.
Tip #1: at least one person carry an auto-shotgun. You can dispatch a witch from the back of her head, aiming down, on the lower difficulties, with a single 8-shot burst. On the hardest, nope, you won't have time to stop around a witch.
Tip #2: get dual pistols akimbo and get the "autohotkey" set of tools to enable an auto-clicker. You can turn 2 9mm pistols into a fully automatic SMG, with infinite bullets, but reloading every 10 seconds. It is more fun than the rest of the game, and saves the main gun ammo for more important things.
Tip #3: If you are going for the hardest difficulty, forget the AI, bring 4 capable humans, and 4 semi-auto sniper rifles. Explanation: the sniper rifle bullet can cross multiple enemies, and can be hip-fired. The AI director will go into "why don't you die already" mode, and launches more zombies than you have bullets. Funnel them into narrow corridors, and let them line up for multi-shots.
#4: Kneeling is crucial.
#5: Molotovs are your best friend on hardest. Immolate your own team to get away from tight spots. No really.
#6: Use Healing packs on hardest only when a person is seeing black-and-white (and they should communicate that). If you run into defibrilators, then you should use the healing pack early just to carry the defibrilator.