-THE WITCH:
If you pounce a Survivor as a Hunter while the Witch has already incapacitated someone and is attacking, you will not be damaged and the Survivor will die twice as fast. You will also get points for the damage she inflicts
Experienced Hunter players normally crouch just out of sight near the Witch, waiting until a player runs up to attack the Witch and pouncing them just before they get a chance to fire. This gives the Witch the potential to do the most damage possibly even killing the Survivor if timed properly.
Most experienced Survivor teams will have three players take up defensive positions while one goes up to "cr0wn" the Witch. As a Smoker, you can strangle the player who was tasked with Witch killing just as he/she is about to crown her. The Witch will be startled, and the Survivor is basically a sitting duck.
An Infected player can melee the Witch just as she looks up when a Survivor gets too close, sending her after them. The Witch has 1000 health in Versus, so any damage she takes from clawing in generally insignificant unless sustained for long periods of time.
An effective method is to have all of the Hunters surround and circle the Witch as if guarding her. Upon confrontation or encounter, the Survivors will be discouraged from advancing, knowing that the Witch is both dangerous and heavily guarded by its weaker counterparts.
The Jockey and Charger can quickly move a Survivor towards a Witch. In the case of the Charger, an even more destructive possibility may occur. If the Charger were killed, then the Survivor would both be freed, and be a likely target for the Witch. If the Witch were attacked before the Charger were, the Witch would both incapacitate or kill one survivor, and free up the Charger to do much more damage.

-IMPORTANT TEAM TACTICS:
The Survivors This game hates lone wolves like no other. You need teammates to cover your back and pull you out of tough situations. There are a few tactics and tips you should keep in mind:

-POWER IN NUMBERS:
As long as the special infected outnumber you, chances are that they will get a combination that can leave you and your team helpless. On the other hand, if the Survivor team outnumbers the Infected team, these are the only outcomes that could possibly lead to everyone being downed.


-A TANK, SPITTER AND JOCKEY ATTACKS:
The players that are not helpless are attacked by a horde.
Friendly fire, such as a poorly-thrown molotov.
The team fails to kill a Witch before it flees.

-VOICE CHAT:
Voice communication is extremely useful and most players will need to use it to ensure survival. It is not a requirement but voice communication is a lot easier than using the message chat and it will help teammates know what is going on around them while normally offering them a quicker rescue response from special Infected and being incapacitated.

-TURN ON CLOSED OR FULL CAPTIONS:
If you turn either of the above on by going into options, the game will say things like "Tank growls" or "Incoming Attack". Although not necessary, this will help a lot.

-SINGLE FILE:
This tactic is best used in a narrow or confined space, such as hallways or tunnels. The players with the most health should cover the front and rear, while the players with the least health stay sandwiched in the middle. There should be plenty of space for two Survivors between each player, so if one is pounced/grabbed, the other players won't be knocked back, and can aid the Survivor faster.

-MOVING:
One of the most important things to do on expert difficulty is to always keep moving forward. On expert, the Director will almost constantly send hordes and hordes of Infected at immobile Survivors. The longer you take through a level the more hordes and Special Infected you will need to face, so move quickly and cautiously.

One very important exception is during a horde attack. Find a spot to cover your back, crouch and do not move! This helps to reduce team damage. If in the open, you can use a teammate's back, so that you each cover 180°.

In Versus mode, if you are not firing a weapon and you are either crouch-walking, walking or standing still, your outline will not be seen by the enemy Infected. This is very useful for sneaking up on a position where a Special Infected is likely to be hiding. Note that this will not apply to Survivors in sight of Infected players or Survivors covered in Boomer bile.


-SQUAD FORMATION:
An effective tactic, especially in tight quarters, is to split your team into a point squad and a rearguard. The squad on point is responsible for eliminating all threats from the front while the rearguard puts the hurt on Infected flanking maneuvers. This tactic is especially effective because your rearguard and team are close enough to you that they can swipe Infected off you before they can deal damage to you.

CORNERED:
Surprisingly, four Survivors crammed into one corner (or closet) works very well. Deploy two of them in a crouched position doing melee while two more stand behind them and unload their weapons, this tactic will get rid of zombies very effectively.

Caution: This tactic must be abandoned with haste if a Tank, Spitter, or Charger is present. You can tell if a Tank is coming because there will be cue music, the screen will shake, and the Tank will make distinctive noises, like the other Special Infected.

THE INFECTED COORDINATE YOUR ATTACKS:
This is by far the most important factor in a Left 4 Dead Versus match. If a Hunter pounces on his own, he will just get killed before he can do any real damage. If a Smoker tries to constrict a Survivor they will just get freed without any real effort, giving the Survivors about 20 seconds while you respawn or recharge and get into position to try again.

The best tactic for coordinating your attacks is to let the Boomer vomit on as many people as he can. Then, have the Hunters pounce from their hiding spots and make the Smoker pull someone away.

If the Boomer vomits on one person, then pounce or constrict the other three who haven't got Boomer bile on them! The one person will be getting attacked by Infected and will not be able to see that other Survivors have a Hunter on them or Smoker constricting them! Also, coordinate your attacks with the Tank. Wait for the Tank to spawn and let him cause havoc. Then pounce or constrict on any Survivors you see lagging behind or running off too fast. If you're a Boomer alongside a Tank, cover some people in Boomer bile and it's pretty much game over. Tanks are hard on their own, but with a horde of Infected slowing the Survivors down, they don't have much chance of escaping.

-SPAWNING:
Most of the time during a Versus match, the Infected team will spawn one Smoker, one Boomer and two Hunters. However, sometimes it will spawn an extra Hunter as a replacement for the Smoker or the Boomer, creating a more aggressive group. This seems to happen randomly, but it appears related to which Infected die and which live. For example, if two Hunters and the Smoker live but the Boomer is killed, the Boomer player is more likely to spawn as a Hunter. Also, in very rare circumstances, there have been teams of four Hunters; this group can cause major problems even for the most experienced Survivor teams.
Do your best to learn the places you can climb to, good ambush spots, etc. Look for climbable spots that players can't reach or for strategic walls you can knock down. In general, the Boomer should spawn ahead of the Survivors so he can properly ambush while the Smoker should spawn behind to pull someone back. Hunters should be near the Smoker or the Boomer to take advantage of the chaos that they create. Obviously this is all situational as there will be times that you will want to spawn behind as a Boomer and run in.

-RUN AWAY TO RESPAWN:
A good tactic for the Infected in Versus is to get the Director to allow you to respawn. When an Infected gets to far behind the Survivors, a message will appear giving the Infected the option of moving closer to the group. By pressing this, you will be in the respawn mode at the Surivivors' location. You can now respawn where you choose with your Special Infected's ability recharged. This is a useful trick to use if you are in a bad position or want a faster recharge of your ability such as in the case of the Boomer. This method does not change your health.

-FRIENDLY FIRE IMPORTANT RULE OF THUMB:
When in doubt hold your fire and use a melee attack.

Easy - Friendly fire does not inflict damage, but friendly fire incidents will still register and disqualify players from earning the Safety First achievement.
Normal - Friendly fire inflicts 10% of the weapon's damage per hit to the Survivors.
Advanced - Friendly fire inflicts 50% of the weapon's damage per hit to the Survivors.
Expert - Friendly fire inflicts full damage to the Survivors. Avoid friendly fire at all costs. One shot from a pump shotgun will instantly incapacitate a teammate.

-WHEN A SURVIVOR IS BETWEEN YOU AND YOUR TARGET:
Hold fire and ask them to crouch. Experienced survivors are likely to do this as long as they know of your location.
If they are crouched, warn them that you are firing over them, and then open fire. Otherwise, they might stand up again when you do not expect it.
Aim high. Dangerous weapons like the Pump and Automatic Shotguns have a wider area of effect than other weapons thanks to their lack of pinpoint accuracy. Doing this will save your team from potential friendly-fire.
When using a shotgun, aim wide. Learn the size of the shotgun's cone of fire and aim so just the corner of your buckshot will hit the Infected. This is usually more than enough to kill a Common Infected, and can be a life saver on expert. Do not just attempt this if you feel even slightly unsure. Melee is still king when it comes to getting Infected off teammates.
Switch to your pistol(s). Depending on what your primary weapon is, your pistols can be more accurate, thus lowering the risk of friendly fire.