Left 4 Dead Achievement Tips

CrownD and Witch Hunter achievements (50Gs) : This requires you to kill a witch in a single shot and not taking any damage. This may seem like a daunting task - but can be done quite easily. Simply, equip the automatic shotgun and locate an area with a witch (You will want to do this on normal difficuly as they rarely spawn on easy) Once you've cleared the area of zombies quitely approach the witch - flashlights off and crouching - and preferrably from behind. Have the target reticule rest on her head. If you are within a close enough distance to her (less than 2ft) she will start to stand up. As soon as this animation completes, fire one shot to her head. And kaboom - 50GS.

Helping Hand (20Gs): Reviving 50 incapacitated survivors might take some while especially when 1)- You are a high risk of dying yourself while carrying out the revive and 2) - You will be competing with 2 other survivors to carry out the revive first. So here is a mini guide to getting the achievement - very, very easily. Load up a campaign on Expert and grab a weapon. Quickly down all four survivors and revive one. When that survivor gets back up, incap and revive them again, before finally killing them. Carry out the same process for the other 3 survivors making sure you incap/revive/kill one at a time (as they will revive each other before you do) After several reloads of the campaign (completing this process each time); you will get the achievement.

Man vs Tank (30Gs) - Load up the last chapter of the Death Toll campaign on Expert. Clear the immediate surroundings (making sure you have enough health to continue) Before you continue down the street and through the house into the grassy area (where the tank will meet you) -kill all other survivors. Make sure you have dispatched of all special infected first as this will cause you problems later on. Switch the difficulty to normal and continue westwards after you come out the garage - up the street and through the house. Grab the pain pills in the house. Upon entering the grassy area a tank should spawn. While keeping a safe distance from it - strafe around it using the whole open area firing shots at its head. You will take a few hits - be sure to use your pain pills. Soon the tank will die and you will be 30GS richer. Video for reference : http://www.youtube.com/watch?v=Mn5i-DODtuU - all credit to achievementhunter.com for this one.