Cr0wnd (Kill a Witch with a single headshot): This is quite simpel if there's no SI around. Walk up to her, wait for her to get a bit angry and look at you. Then shoot her in the head with the shotgun (auto is your best bet).
Untouchables (No Survivors take damage after contacting the rescue vehicle): Can't really tell you how I got it, but I've seen a lot of people just getting by joining the server at the very ending of the finale. The easiest way is probably on easy with a few friends (kill the bots if you're not four).
Do Not Disturb (Sneak past all Witches in a campaign without disturbing one): This requires some luck - that is, sometimes the witch is placed in such a way you can't sneak past her. Also, don't use bots for this one.
Man Vs Tank (Single-handedly kill a Tank): Just play with some friends on easy that you can tell not to shoot at the tank.
Safety First (Play an entire campaign with no Survivors taking friendly fire damage): Play a easy campaign with bots where you only use mele. The bots can easily handle the infected.
Nothing Special (Survive a campaign with no Survivors taking damage from Special Infected): This one can be really tricky. You can do it alone with three bots by going fast throught the levels, but stop up and stay close to them when you hear a SI. Blood Harvest is a good map because it has big open areas. I tried this two times, and both times Louis was hit by a rock by the last tank in the finale. I ended up doing it with a few mates killing the remaining bot. Just move fast and stay close to each other when a SI is around. Actually, I found the hunter to be the worst. On our first try a hunter landed in front of a mate in the finale and hit him.
Stomach Upset (All Survivors complete a campaign without being vomited on): This can easily be done with the Nothing Special achievement.