shot, kill, reload, repeat

TANK:
Being a tank doesn't mean you have to go into "HULK SMASH" mode. Against a competent team a tank is only as good your other zombie teammates. Wait for your team to respawn and aim to incap at least one, who you then proceed to guard.

If any other special infected caught a survivior, DO NOT PUNCH THEM. You'll kill your fellow teammate and let survivior loose once again.

Play defensively as a tank. Protect an incapped survivor and let them come to you. You won't lose control as long as you maintain a line of sight and the incapped person will bleed out unless the others do something. If you suspect they'll throw molotov at you, be sure to stand right near incapped survivior - fire will quickly finish them off, and team will be crippled for the rest of the chapter.

Note: Guarding does not necessairly mean "stand on top of them bashing away". You do not do enough damage to kill them effectively. Incap one, find some cover and smash those silly survivors who come and rescue them (preferably with a car!). On that note, you are bulletproof. You can stand in between a pounced survivor (punced means with hunter on him) and their teammates and they wont be able to shoot the hunter off.

On other occasions, avoid fire and chasing survivors into vents/enclosed spaces at all costs. Tanks climb on ladders extremely slowly, they also walk crouched extremly slowly.
DO NOT chase green surviviors. Person above 60 HP (fgreen) WILL outrun and decimate you.
DO NOT use right-click attack as a tank. It is extremely slow and easy to avoid. On avoiding AI tank rock throw - he throws rocks ahead of you to hit you when the rock reaches you. Precisely the moment the rock leaves his hands, change the direction you are running at, and it will miss.

Weaons damage to tank chart here: http://images.wikia.com/left4dead/images/f/f3/Weapons_effectiveness_on_tank2.jpg
Most effective: ak-47, combat/tactical shotgun, M60 - all below 20 seconds to take down tank on versus. Of course, melee weapons and chainsaw are most effective.

Chasing tank with chainsaw/melee weapon behind his back on VERSUS ONLY is not a bad tactic, it is hard to notice during a chaos. DO NOT do this to AI tank, he WILL punch you.

You can stop tank on a ladder. He won't be able to go up, and you'll get free melee weapon hits on his head (hurts very bad). Works very well on AI tanks, unexperienced players fall for that too.


WITCH:
While no longer terrifying, she is most probably a free incap. Do NOT let the survivors get within melee distance with a shotgun. Hitting her few times in the head results in her immediate death, and this tactic is called "crowning the witch". Alternative tactic that works only on EASY and VERSUS, is shot her in the head with SNIPER RIFLE - or any weapon with EXPLOSIVE rounds - then run up to her, push her, hit with melee, push, hit... she'll die after 4 hits.

To set off witch as infected, you first have to make sure surviviors are within her range - means she stops crying and eyeballs the survivior with a gasp. The moment you hear that, run up to her, hit her ONCE and she will startle. This ONLY WORKS if at least one survivor is still inside her detection range when you melee her, so speed is the key.
Likewise, a boomer explosion will only set the witch off if she has already noticed the survivors and if at least one survivor is still within her detection range. Standing by her while the survivors are xxx feet away will only get you killed to no purpose. As a boomer, still remember you can only slash her, and run up to surviviors to cause more chaos!

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Also, Hunter, Jockey and Charger can steal Smoker's victim while he is still pulling it. It is a very good tactic to have smoker pull someone to line him up for a insta-death charge. Don't rage on someone "stealing" your points/kill, they might've had some great idea in mind that didn't work.

Be creative, and communicate with a team!

Last but not least important: THERE IS NO AFK IN THIS GAME. NEVER DO THAT UNLESS YOU ABSOLUTELY HAVE TO (diarrhea, for example)!