First off, I can't guarantee that these cheats will save each of your teammates. The apocalypse is dog eat dog, so the following tips are to insure your own survival.
But these are secretly obvious ways to combat the attacks of each type of special infected.
Hunter: Zig-Zag Maneuver! Once the Hunter leaps, it cannot change direction(mid-air). If you find yourself spread out from your teammates and spot a hunter coming after you, start moving in random, jagged directions! It takes some getting used to, but you can run back to your team(and have them kill the Hunter) or multitask--zigzag away and shoot the Hunter simultaneously! Kickass!
Smoker: They are not known for being fast runners. Specifically, if you find yourself snared by his tounge, and a nearby teammate saves you--but leaves the Smoker alive(by melee shoving you or shooting the tongue)--RUN AFTER THE BASTARD! The Smoker is paralyzed for a few seconds after his tongue is broken, giving you a headstart after him.
To prevent being snared in the first place, run directly in front of a Smoker. Since he depends on his tongue to drag you long distances, if you're close to him, he is useless and attempts to run. Shoot his retreating behind!
Boomer: What's the worst it's gonna do? If you're bound on killing fatties, just run up and kill it. Melee Weapons and Shotguns (hell, even Magnums) only take one shot to kill the Boomer. They are, however, very prone to stumbling, so run up and shove it away, back up fast, and shoot. How can you miss?
Spitter: The Spitter stands taller than other infected and run in a peculiarly different way. This makes them easier to spot.
ALSO, a neon green trail of spit come from their mouth while they run. It means they stand out.
The easiest way to kill these revolting creatures is to run up to it as soon as it come on screen, and as you (nonstoppingly) run by it, use a melee weapon kill it. So long as you keep running, the acid that forms upon a Spitter death will not have spawned yet, and consequently, not harm you.
Charger: I love Chargers. Like Hunters, once they decide on a direction to charge, they cannot stop or turn. So man up, take the bull by the horns, and stand in the Charger's path (perferably a few feet away, giving you time to move). As soon as you hear the Charger's signature ARAAWWWWWWWWWWWWWWWWGH yell of "I'm Running!", take a few steps to the left or right. He'll just keep running further away from you.
ALSO, another way to protect yourself from these baby-tanks. /GET TO HIGHER GROUND/. A Charger, if hellbent on destroying you, needs to get to your level before it can charge. However, the AI of the infected is not perfect. Depending on the number of brain cells in his head, the Charger may run into whatever you're standing on, instead of you, and will stumble. So when you hear a Charger, hop up on a box, a car, hell--even a stairway railing will work. The Charger will either A) run into the support below you and stumble, B) attempt to climb up (you can shoot it to death while it's attempting this), or C) see you as invincable, thus targeting a different teammate. Hopefully Coach. ;)
Jockey: You can hear these guys a mile off, but I digress. When you hear the maniacal laughter of a Jockey, gather with the rest of your group. By bundling closely together, if a Jockey succeeds in grabbing one of you, the free teammates are close enough to melee shove the Midget Zombie off of your back in a split second. Success? I believe so.
Tank: These guys are easier than you think. It just takes a little mindpower, good ol'weapons, usually a sacrifice, and the right game settings. Once the campaign begins, pause the game, go down to "options", select "audio/video", and scroll down to where you are given the option for subtitles. Set your game to the option of "Full Captions". Best. Creation. Ever.
This will give you an on-screen warning of each infected (it is color coordinated. For example, when Nick is speaking, the font is blue. Rochelle's font is Pink, etc...) before you even hear it.
The most helpful thing about captions is that it will warn you about a Tank before you even get close. Specfically, the caption will say [Tank Growls] before you enter a room or turn a corner. This is a sign to be on your toes.
The Tank will not come after you until you trigger it by entering the room he is in, waiting. Sometimes this means opening a door, running into a room, or getting the Tank on-screen.
So if you're being sneaky, run into the next upcoming room, and run back quickly. This will signal the Tank to come after you, so you and your team can shoot it as it chases you. Fullproof.
This cheat also gives you an opportunity to get a molotov ready. Enter the next room, throw the mollie, and get back. It will set the Tank on fire, and thus send him after you.
--NOTE: THIS CHEAT DOES NOT WORK ON FINALES. IN THE FINALE CASE, THE TANK WILL JUST COME TO YOU.--
Witch: Similar cheat as above. If your hearing isn't superhuman, use the Full Captions. Any font in Purple is either Coach speaking (dark purple) or a Witch Moaning (bright purple).
Specifically, the game will give you details on the witch's mood. From a distance, you read "Witch Moan". As you get closer, you can read the in-game hints of;
"Witch Groaning - Soft" -- She's probably in your proximity. Set a direction you KNOW is clear (perhaps the way you came) and then look for the witch. Or, keep running.
"Witch Groan - Loud!" -- RUN LIKE HELL! You're really close to her and she's going to attack whoever is closest. Back the HELL up, and hope she takes the other AI teammmates.
"Witch Surprised!" -- You probably got a little cocky with your bullets and shot too close to her. She will not become startled (so long as you move away), but she is not aware of your being there. Immediately after "Witch Surprised!" she will change to the status of "Witch Groaning - soft" And become progressively louder when you refuse to move.
"Witch Retreating" -- Retreating is what a Witch resorts to after her target is dead or out of reach. EX: IF she become startled, and that player then throws themselves off a building, she will recognize the death and run away with her hands over her face. She cannot become re-startled during this phase.
"Witch Shriek" -- Ok, who shot the bitch?
"Witch Frenzy!" -- In this case, you or another teammate has been pinned by the witch and is being currently slaughtered.
ALSO: A Witch can re-target people once she had shrieked. Especially if she starts running at the asshole who shot her, and a different teammates lights her on fire with a molotov cocktail. In fact, if anyone set the witch on fire after she's been startled, she will retarget to the person who lit her up.
HINT: Explosive ammo will force the Witch to stumble--a lot--and give you more time to shoot her. Deploy explosive ammo into a gun with lots of capacity (like an M-16 assault rifle [50 bullets], a SCAR [60 bullets], or even the M-60 exclusive to the passing [150 bullets!]), and start firing at her. She's likely to die before she even heads after you. The downside to using shotguns means you have to relatively close to her in order for the extra ammo to take much of an affect. And you don't want to accidentally be too close!!!
I hope this helped someone somewhere. Thanks for reading it, everyone. 8D
NOW GO KICK SOME ASS, YOU SPECIAL KILLERS!!