Left 4 Dead 2

I did this single player.
Start up The Parish, chapter 3: Cemetery
Do as the video says. I did a little differnt since it did it with 3 bots. Once I got to the impound lot I ran away from the cars and had the infected run to me to kill alot of thems so the bots wouldnt trip the alarms on the cars when they were shooting. Then when most of them were dead, I follow the video staying on the white cars. Once through the impound lot just finish the level and make it to the safe room. It will pop as soon as you get into the saferoom.
Video credit goes to RoosterTeeth

My friend had Left 4 Dead 2 before me, and he said this achievement to me and at first i thought, 'Thats impossible'. But when i played, i only noticed that there is only a few cars. But this achievement is so much easier than it looks. All you need to do is, grab a melee weapon (there should be one right behind you as you climb up into the area) and only jump on the white cars, do not go anywhere near the red ones, plus, make sure there are no smokers around, because if there are, they make drag you into an alarmed vehicle. Just make sure you use a melee weapon, because if you use a normal gun theres a slight chance you may shoot the alarmed cars. As soon as you walk into the safe room, the achievement should pop up.

Easy way to do this is load up Chapter 3 of The Parish using the Chainsaw Massacre Mutation. Chainsaw do not seem to set off car alarms and the AI won't do it by accident. Just go slowly and make sure you don't jump on a car with an alarm. Achievement pops when you hit the safe room at the end.

I did this drunk as all get out. Start a single player game with a melee, and run run run. In the section with the cars, white cars don't have alarms, the red cars do. Run your way through that section as fast as you can. I've noticed the more left behind you bots are the eiser these sorts of achievements are. Let your bots catch up and do what you want after that till you reach the safe room. Too easy.

You'll need to use a melee weapon when you enter the car park compound, as to not risk any stray bullets hitting the flashing cars. Also the achievement won't actually unlock until you're all standing in the saferoom, so keep that in mind. This achievement is actually easier than it looks, just look at the cars and make sure the path you choose doesn't have any vehicles ready to alert the hordes.