Surviving Large Hordes
Go into a corner with a melee weapon ready. Then, press and hold down the Right Bumper button, and aim at the incoming hordes.
Use an adrenaline shot, followed by a medical kit.
Tips And Strategies
For the regular, common infected zombies, try shooting them in the head with your secondary weapon, which would be either pistols, or a melee weapon. It seems to be a waste of ammo to use your primary weapon.
Try using the chainsaw melee weapon, at least once. It’s pretty cool, and it can rip through almost any member of the special infected.
If you find yourself being charged at by a lot of regular infected, then find a nice long thin hallway, preferably one with a door, and get behind the door. The regular infected zombies will have to knock down the door first, before they can get to you, buying you precious time.
Kneel, so that your teammates can fire over you, from behind. Also, with the first player kneeling in front, the second player would be a whole lot safer, behind the first player, than not.
Look for higher ground that you can get to, preferably something that zombies will have to move a long distance in order to get to. Also, when on high ground, it makes using the grenade launcher more enjoyable.
Try to shoot from behind any kind of cover that you can find, whether it be from behind a window, a doorway, a long thin hallway, or even inside of a truck.
It’s almost always a good idea to keep moving. Zombies will eventually attack you again, so standing still usually isn’t very smart. Also, a moving target is harder for the special infected to hit.
Unlike the first Left 4 Dead, the shove takes a little bit of time, before you can use it again. This makes using your other weapons the more ideal option.
Whenever you get the special ammo, try to use it sparingly. You might try saving it for the special infected. It runs out faster than you might think.
I personally wouldn’t bother with the defibrillators, in the campaign mode. Your teammates will eventually respawn in closets, so grab the medical kits instead.
For the boomer, shove him back if he’s close to you. Then, shoot at its belly, from a distance. Its belly is easier to hit than its head, but if you can headshot it, then go for it. Try to make sure none of the survivors are near it, when it dies.
For the smoker, it won’t wrap its tongue around you and pull you in, if you’re close enough to it. Instead, it will claw at you, and attack you that way. If you’re really close to a smoker, then use the shotgun. If you’re further away, then try finding an object to hide behind, when it lashes its tongue out your way. It takes some time for its tongue to go back in its mouth, and the smoker becomes vulnerable to your attacks. Dodge its tongue, then kill the smoker.
For the hunter, if you can shove it, while on the ground, or even in the air, just before it gets to you, then shoot it while it backs away, and your chances of killing it will greatly increase. If you can trap it or corner it, then your chances of killing it will also greatly increase.
For the tank, shoot at it, either with a shotgun, an assault rifle, or a grenade launcher. Out of the three weapons, the shotgun seems to be the best choice, but one good hit with the grenade launcher will greatly weaken it, to the point where you won’t have much trouble killing it.
For the witch, if you can’t avoid it, then try sniping it in the head. Next, it will usually run at you in a somewhat straight fashion. When it gets close to you, throw a Molotov at it, then back away, shooting it down with a shotgun. The best thing to do, however, would be to avoid the witch altogether.
For the charger, if you get close enough, he won’t charge at you. Instead, he will attack you with his huge arm, and he’ll be easier to kill. I would recommend using the shotgun on it, but a few pistol shots will kill a charger.
For the spitter, I would recommend going for a headshot, but all the spitters I’ve ever had to deal with have been surprisingly weak. Kill it, basically, with any gun, and make sure to avoid its spit. If you find yourself in the middle of its spit, then jump, and run away from the green goo, to help save your health.
For the jockey, move away from it as it approaches, constantly shooting at it, and your chances of killing it are pretty good. Pistol shoot at it from far away, and then shotgun it down, at close range. Either that, or use a sub machine gun, or an assault rifle, to put it out of its misery.