Left 4 Dead

This is a fairly easy achievement on either normal, or easy difficulty (of course setting it to easy will make the achievement easier, not just for the zombies but mainly for friendly fire.)

I suggest doing your favorite level with some friends who are good at the game, and preferably who also like the level you're doing and are familiar with it.

Try not to rush ahead, or lag behind as you will take damage from specials if you do so. Instead try to stay in the middle of the pack and let your teammates push through the level. And protect your back.

Make sure to be on the lookout for pain-pills and med-kits and use them if needed. Pain pills should only be used in yellow health (45 or below) and med kits can be saved for red health, although I would suggest if you're going for this achievement to use the med kit at 25-30 health or lower (that may be red but I haven't played in a few days so I don't remember)

When the tank comes I would advise trying to just stay out of it's way and let your teammates kill it, 3 people can easily take out a tank on the lower difficulties, and two people could take out a tank on easy with no problems.

Also make sure to avoid the witch as it's an INSTANT incap (instant kill on Expert) let your teammates kill her if it is a witch you can't avoid.

As for the finale doing a level you're familiar with will give you an edge as you will know where the specials tend to come from. Doing a level with a "Jesus" spot may help, but is really not needed for the lower difficulties.

i had no trouble doing this on the death toll with human players. just make sure that you are diligent about using all the pain pills and med kits you find. oh and dont be afraid to not rush into things, its not a race. lastly, as soon as you hear that the boat has arrived, get over there no matter what, and dont stop to help other people. just make sure you get there alive, otherwise the whole hour was wasted

You can get this at the same time as 'Unbreakable' if you're good, start up Death Harvest on easy, be calm don't run off by yourself, make sure that the bots are following you and as soon as a horde or a tank comes be very close to the bots, if you do this you should be alright.
Make sure to use a lot of pain pills if you need them.

If you've spent the points for Crash Course, this becomes even easier as that's only 2 chapters but still counts as a Campaign for achievements like this and "Do Not Disturb" -- less chances to screw it up.

I guess it would be best to do this on easy and maybe on your own. The AI is usually good with healing you so you should have no trouble here...just make sure you kill before you go down.

Play versus with another control, have the other control be the infected but never spawn, the player will be the only infected to spawn in the whole campaign, I got this plus 5 other achievements at the same time!

try to avoid the witches, i did actually avoid them all when i got the achievement so im not to sure if they count.