Received the achievement for beating a campaign, while sneaking past all the witches.
I also attempted to get the achievement for killing a Tank solo...but I wasn't able to. I don't really understand how you're supposed to get this. There's always people with you (multiplayer and solo). I tried to do single-player and kill off the bots that are with me (so they can't shoot)...but than I kept getting hit by Hunters and Smokers. Ugh. I mean, sure, I'm careful...but those things can come from behind at any moment with little ability to see it coming. I dunno, I'll get it eventually, but it's quite annoying.