During the first mission of Chapter 2, New Beginnings, you will need to complete a mission in Marik's Marina in which you will gain access to boats. After that, you can drive one whenever you like around the swampy area of the Flooded Jungle. In this swampy area, you will find many Drowners and a few Walkers here and there. You must kill 100 enemies by using the speed boost (by holding down the A button) and running them over. If you hit them while the speed boost is active, you will always kill them in one hit, even for Special Infected such as Floaters and Thugs. If a Drowner grabs onto the side of your boat, just boost for about 2 seconds and it will kill them and still count towards the achievement. This is a long process, so you can either try to get this by naturally progressing through the game or by just continuously driving around in the boat, as the zombies do respawn. I did find that if I came back later on, there would be much more zombies, so if you've been driving around for a while and find that the amount of zombies has become sparse, I advise leaving and coming back a bit later and there will probably be way more.
I did run into a problem with this (though my co-op partner somehow did not) where it wasn't counting my speed boost kills towards the achievement. The best solution I came up with was to keep holding A a bit after you kill an enemy to make sure it counts, as I think I may have subconsciously let go of A before hitting them. It still killed them, but it wouldn't count. As well, I had a long stretch of my kills not counting, but when I switched to a different boat, it suddenly counted them again, so you can try that too if you're having issues. You can always check your progress towards the achievement by going to the Achievements area in the Start menu.