Instead of running the levels over and over again I wanted to find a way to idle this achievement. It's not too hard to set up, but it is fiddly until you get the right balance.
In Fields of Terror there are two tractors, one off to the left of the start of the level and one in the shed/fuel storage at the leftmost marker.
At load screen choose single player NOT score attack. You'll not want to have to worry about timing out as this method does take hours. At the start of the level you can pick up the additional gun provided or head directly to tractor down the dirt highway. I suggest picking up the gun as well as the ammo there as you'll have to need them to fend off the zombies while you're perfecting your trap.
Once you have the gun, ammo, and tractor drive down to the shed. Wait until the red barrels spawn and blow them up. Take it from me if you don't do this the zombies can push them into the tractor and kill you. Then you'll want to position the first tractor along the wall so the deadly bits are blocking the door a little bit.
Fend off the zombies as needed but DO NOT break the vials in the Mutation Station. If they all get destroyed the station will close and the light will disappear and you'll have to start the level again. I've blown up the barrels all around the station as well as slammed the tractor into the station and at least some of the vials survived.
Inside the building is another tractor, this is the one that you'll be sitting in and getting all the kills from. Now once you're inside the second tractor you'll want to maneuver it so that the tractor is at an angle and there is a small walkway between the two blades of the tractors. Think about making a 'v' with the blades.
The walkway is the key to having this work, if the zombies are spawning but aren't running towards you then there isn't enough room for them to get to you and their AI shuts down. If the zombies are getting through then the gap is to wide. You can either get out of the tractor to kill the zombies or stay in and maneuver around to do the killing. Keep in mind that the zombies will knock the tractor around if you let them so you can't just ignore them; but they can't kill you while you're in the tractor.
It'll take a bit of time to find the best angle and gap for this to work, so before you completely idle check back every 5-10 minutes. Don't be surprised if you come back and you're in the tractor and staring at the back of the building to just straight ahead and the zombies aren't moving. Like I said this is the fiddly part, and the extra ammo is nice to have.
Once it's all set-up it's loud and you'll have a very boring view. The wait is long so if you can switch screens to watch or do something else.
If the game crashes you'll lose the kills and it'll look like your tracker has stopped. Don't fear, Xbox just can't handle the loss of stats, it's just reverted to a previous number. Keep at it and the tracker will start up again. Or if suddenly the zombies are getting to you and not dying when you hit them with the blades of the tractor don't worry and just restart the pitch. That happened to me and I don't know what happened but it was fixed after the restart.
I didn't keep track of the time I spent doing this but it was several episodes of Criminal Minds.
From Lord Valoel
Perhaps I’m a bit late to the party on this one, but I have some number crunches just in case. If you setup both tractors at the barn near the very back of Fields of Terror, and sit yourself on the back door, you’re looking at about 8 kills every 45 seconds or so. To make its bit easier, that’s 32 kills every 3 minutes or 640 kills every hour. Since this is out of 5,000 kills, every 50 kills equals 1%. So using this method, you’re looking at about 12.8-13% an hour if idling. This is only using one tractor though and letting it idle
From Petrolium Bear
Just some friendly advice, it did work better when I done this at the `Red Barn` and parked blocking the small door at the rear of the barn. I could not a good setup doing this with the Lubertine shed in the pictures in the solution.
*Thanks to Spliner for the help with the screenshots!