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.