This one isn't too hard as you should be using a vehicle to rack in the kills for your 100K+ kills, and the 300K+ kills required by the prestige challenges anyway.
The go-to would definitely be the RollerHawg for its speed, power, and flames. If you are stuck or unsure where to go to maximize your efforts, use this picture to guide your route:
Thanks to JACK LAYT0N for the picture.Make sure to clear cars out of the way and your route will become maximized before you know it helping you snag the high amount of kills you desire.