This method only works if you have the Crash Course DLC (and a 2nd controller)
This was the easiest method I found to get the kills quickly without actually having to play or tape down buttons etc... You just have to leave your xbox on for a few hours. Obviously you can also get
Load up a versus match on the Crash Course finale, with yourself on the survivors and your second controller infected. Just rush through to the generator section with your main profile. Now you want to practice climbing the generator a few times. You just have to stand under the light bulbs and jump up the pole. Keep jumping until the bulbs/light itself is at eye level.
Once you can confidently climb the generator, turn the power on and get climbing. Your second controller will soon be given control of the tank, ensure you walk it over to the survivors and get killed before you lose control. This will happen once more before the generator dies.
Jump down when it's as clear as possible and re-start the generator, and get climbing again (quickly!). Now the 2nd controller will be given a tank again, this time go and kill the AI survivors, who by this stage should be in the back of the van. Once they are dead just stand the tank near enough to your main profile, so you don't lose control.
Now the horde will just continously spawn and attempt to attack you, if you are positioned correctly ( you may have to play around with this a little bit, dead center seems to be best) they will fall over as they try and climb towards you, although you aren't killing them, they do count towards the achievement. Now just put your controller down and go to sleep or watch TV or whatever (watch the screen for a while just to make sure you aren't being hit). I'd estimate about 6 or 7 hours maximum, it may actually be much much quicker than that.