Left 4 Dead

This will come in time if you spend enough time playing Versus.

For the most part, boomers will get you the most amount of damage, especially if you hit 3 or 4 of the survivors with the bile.

A few other tricks:

As a smoker or hunter, stand near the alarmed cars and other like panic events. Most survivors'll see you and automatically fire at you, possibly hitting the vehicle. Quickly dodge behind the car or jump away as a hunter, the horde should swarm them, and then either tongue or pounce them. The attacks of the horde will gain you damage as you hold them down.

Similarly, using teamwork, a player covered in bile makes for an easy grab, and if they're being swarmed by infected, any damage dealt will add to your total. I personally have dealt nearly 300 points worth of damage just from this tactic in a single grab on two seperate occasions.

As always, look for stragglers or people who run ahead to get some damage in before the others can catch up. The infected take no damage from falling, so use that to fall from high heights and catch them from behind for a quick attack.

A solution I made because for some reason using the Boomer strategy didn't work for me. I started this achievement today with around 400 damage already dealt and got it in a couple of hours. I also realized that you don't get up to 500 damage points per round as I say on the video; it's closer to 400, but that still takes at least 12.5 rounds

My Method is slightly similar to TheRickMoranis. It will net you 500 survivor damage in 7-10 minutes depending on how fast you are.

Load up a split screen versus on No Mercy Finale. Start playing as your second controller (the one not getting achievements). Pick up a health pack in the beginning and make your way to the stairs out to the rooftop. Now before you go up the stairs, kill all your teammates. (if you bring the auto shotty it goes faster).

After you have slaughtered your annoying team it is time to get to work. Call for help and fend off the first wave by yourself. When your main account is becoming the tank, put your survivor in the room with the health packs. Now you can punch the survivor as the tank 3 times (be careful not to down them) and heal...and repeat 5 times using the health packs on the wall. when they are out of health packs, punch them to your hearts content and the round will end. Call a vote to go back to the lobby and repeat!

For this you will need 2 controllers.
Connect the 2nd controller and choose Versus Mode:Play With Friends, then Create Lobby/Versus Mode.

Choose the No Mercy campaign, Chapter 2:The Subway. Make player 1 a infected. Player 2 needs to be a human.

Start the Game.

Now as player 1, the infected, go trough the red safe room door and look to your left, there's a pipe you can climb.
Climb up and stay there without spawning.

Now with the second player jump down the hole in front of the safe room and make sure all your A.I. team members are down the hole. Now turn around and there should be a piece of pointy rock that you can jump on to get back up.
Go back into the safe room.

Now grab the first player's controller (the infected) and spawn.
Run inside the safe room as fast as you can without the AI shooting you. Now you can hit the second player without the AI interfering. After you killed the second player and the next round has started, press start and select Exit to main menu.

Now rinse and repeat!

sign in another controler as a guest and make a privite game. then be a zombie. first youll start as a smoker, then a boomer, then a hunter, repitivly. so then get your guest to lead the survivors into a room near the door, but not close enough to shoot you. get him to kill you untill you are the boomer. puke on all four survivors and wait outside the door untill your puke refills. then repete. the normal zombies that do damage to the guest count as your damage to the survivors as well. so when the survivors are dead do the same thing in the next chapter. and the next. you can get all of the multiplayer achievments this way. it just takes a bit longer.

What i did here is to set up a private versus game with a second controller plugged in and start the very first level of No Mercy. Your primary account will most likely spawn as a Smoker which is exactly what we need here. Go up to the top of the building next to the spawning zone, then grab your secondary controller. (Don't worry about the warning that you can't spawn there until the survivors leave the safe zone. Eventually you will be able to spawn in the very location) Pick up a firt aid kit and kill two of the survivors. Now it is time to grab the AI survivor with the smoker. Just before it goes down shoot it's tongue with your second controller (who is the survivor) and heal up the AI. Again, grab the AI survivor with the Smoker and finish him/her off. After that you kill the secondary account. When the round ends, call up a vote for return to the lobby. It is still a slow process, but with healing the AI it gives you more opportunity to deal greater damage per round.