This is actually a fairly difficult achievement. It's for completing mission 31. You'll have 1:50 to get from one side of the map to the other.
There are a ton of green bomb zombies, I recommend using the bus and make sure your rail gun is fully upgraded at this point for sure.
You'll want to go all highway up to the end here. Look at the map in the beginning. The route I took was to the right. Follow the highway all the way to the right and then it will loop back to the left where the main street is. Take a right on the main street and get ready for a load of zombies.
At this point make sure you have at least 3 rail gun shots left as you have to make it through a mass of not just zombies, but lots of exploding ones as well.
It may take some practice, it took me several tries. The last time I did it well enough I didn't have to stop for any health which left me 30 seconds still on the clock. So if you need to stop for health, there is some on the way in several different spots.