@sublime78, I got "Do Not Disturb" playing No Mercy on easy with the AI. There was a bit of luck involved. The director's placement of the 3 witches weren't in the middle of the path so I just ran as far away from the witches as possible and the AI followed.
The achievement that's pissing me of is Cr0wned. I swear I get the witch's attention every single time and fire right at her head with the auto-shotty and yet it never kills her in 1 shot, always 2.
I tried playing expert with the AI and it's a joke. I can get out of the subway in no mercy but the AI is worthless after that. The 5 hardest achievements in the game (Untouchables, Unbreakable, Zombicidal Maniac, What Are You Trying to Prove?, and Nothing Special) you need 3 good, dedicated human players.
Also an easy way to get "Back 2 Help" is start No Mercy on expert with AI. Get to the first safe house and quickly run in and out, run far enough away from the safe house so the AI will follow. Then kill all infected around, when its clear, shoot one of the survivor AI with the shotgun, help them up and return to safe house. That's how I got it.
@ Munkeh111, there's about 450 easy gs, 350 more that's a challenge and the remaining is hard.