Left 4 Dead

Just select the last chapter of the no mercy campaign on easy (best is to just do it in single player with AI, they will stand behind you and kill the infected hitting you in the back) and just rush straight towards the end make the 2 calls and run up on the roof and get on the turret. Kill the first wave of zombies with the turret until the tank comes, kill the tank and get back on the turret to kill the second wave of zombies until the second tank comes. You should be able to get around 100 kills each time before the helicopter comes. There are other parts obviously with turrets and lot of enemies, I just prefered this one.

Load up the DLC "Crash Course" finale. Then play through the full level until you get to the generator. Start it up once(ONLY ONCE). Then kill the two tanks that will spawn. Once that is done the generator will kick off (AS LONG AS YOU DON'T RESTART IT IT'LL SPAWN INFINITE AMOUNT OF HOARDS OF ZOMBIES) Then jump on the turret behind the rescue vehicle (its on top and to get on it jump on the two cardboard boxes). Then just spray zombies for about 20 minutes and it's yours. Pay attention to where the hoards spawn as you'll rack up the most amount of kills, killing the ones coming from the fence to your left and the crates in front of you.

-TIP if you select a molotov, pipebomb, or health kit before you get on the turret it'll make your cross hairs like they would be as if you hadnt got on the turret which improves the accuracy just a tad.

;D if negative vote just let me know how i can improve my solution. Good luck sprayin' down ZOMMMMBIEEEEZZ!

No Mercy's Easy finale really is the way to go. Just keep playing, always on the minigun, with either AI or your friends, and just watch it go up.

If you go in the pause or main menu of Left 4 Dead and press 'Achievements', you can see how far you are to getting Red Mist.

Survival mode's Death Toll - The Streets also works well.

Hope this helps.

On the boathouse finale, as long as you don't get onto the boat and escape, you are free to stay on the balcony where the mini-gun is located and just keep blasting the horde...be warned, depending on how long you stay there and what difficulty you are playing on..things get very very difficult.

Play with AI as they are pretty good at protecting your back. However when you see that tank coming, I suggest getting off the gun and walking around the awning of the house. Hopefully the AI will blast the tank and then get straight back on that gun.

Won't take you long.

You can get this even faster, if you choose the Survival Mode and start Death Toll in "The Streets" chapter.

You'll start right next to the turret. Just make sure to kill the Tank between the Waves(The AI-Bots should take good care of enemies in your back). You can get a multiple hundred kills per run.

Help out the AI, if they have trouble with special infected, so you can go on even longer (besides the horde is right in front of you, then just stay at the turret and kill anything that is in your way, another AI could help the troubled survivor)

It just took me a half hour and I started with less than 50 kills.

(You can also start any other chapter with turrets, as you'll spawn right next to them in Survival Mode)

P.S. When you die, don't go back to the lobby, just let the game going since you'll start right away after that. (saves time)

P.P.S. depends on skill though how much you get out of each round, actually even more easier with 2-4 Players. Good luck!

Just repeat the Runway Finale of Dead Air on normal to get this.
I got 100-150 kills each time i played the level. Just make sure you pay attention to whats going on behind you, or have a coop partner cover your back. Using pipes bombs to bring lots of zombies in front of you is also helpful. If a boomer comes, try getting covered in bile because then more zombies will come for you to kill.

With the new Survival DLC, just make a private match set in No Mercy, with Survival settings of course. Since you spawn right below the minigun and probably die as quick... (I've gone over the 4 minute mark solo). You could rack up the kills when one of the AI gets vomited on by a Boomer.

Just rinse and repeat till you get it. Also watch out for smokers.

Another thing, the tank will usually appear behind you around the 40 second mark if you still pluggin away with the gun... so if you don't see the tank when the screen/vibration happens... you'll know he'll be there.

Select the last chapter of No mercy and play on easy(i did normal but its your choice). Grab a pipe bomb at the begining and make your way to the holding place. Call in the chopper then get on the roof next to the turret. Once you start seeing zombies throw your pipe bomb in down between the 2 buildings in front of the turret. Then use the turret to kill the zombies that go to the pipe bomb. This probably adds 30-50 extra kills. Then just sit on the turret the rest of the time untill the chopper comes. Don't forget to get off when the tank comes or you can get screwed over. Also another suggestion is to get yourself killed at the end so you can just restart the chapter instead of having to go to the menu and loading it up again.

I suggest playing on easy with yourself and a friend. Playing on easy will allow you to friendly fire without causing any damage to your team-mate.

You can do this on any mounted gun, but I suggest No Mercy.

Have the account you want the achievement for get on the mounted gun (of course).

Have your friend stand at the bottom of the ramp, in view of the mounted gun. When boomers come have your friend get vomited on, or exploded on so you can kill the horde that comes, netting you some more kills. This should allow an extra 20-40 zombie kills per vomit.

The second player (at the bottom of the ramp) can shoot the zombies that start attacking the first player (on the mounted gun) and keep them off their back while the first player can use the mounted gun and protect the second player.

Using this method will allow you about 100-300 kills on the mounted gun depending on how many boomers come and the horde that will come after they vomit.

After the second tank comes have each of you jump off the cliff so you can rinse and repeat this method until you get the achievement or are bored of doing it.

NOTE: This could also be done no the Crash Course campaign. If you do not restart the generator than there will be an endless horde of common infected and you can stay on the gun as long as you want without the threat of specials, or tanks.