This is a tough one two get. I think No Mercy is the easiest to do it on. Get together with 3 friends. Stick together and help each other out. Remember only one person has to make it to the safe house. My tip for part 5. There is a closet to the right of the ammo at the end. All four survivors get in. Two crouch down in door way and start melee as soon as you call for help. The other two stand behind and shoot the zombies being melee'd by your team-mates. When the tanks come run out of the building and throw a moltof at him and then run. Don't try and shoot him he'll catch up and hurt you. If he gets to close to a team-mate jump on the turret, he will instantly go for the person on the turret. You can also try to get him stuck on the ladder. You do this by climbing a ladder while he is following you and stop on the edge of ladder at the top. If he follows you will be blocking him from getting up and of the ladder. Just look down and watch him burn or soot him. Or your team-mates can shoot him. After second tank is dead get to the hela-pad the helacopter is here. If some one is down or getting pulled away LEAVE THEM. Only one person has to make and everyone in the room, even if idle, will get the achievement. WARNING!!! Be prepared for 3-7 hours of gameplay, because you will die alot and have to start at the safe-house again.