This will likely unlock this just playing through the game, but this can be farmed easily in a pursuit as well.
A good car to use for scoring easy takedowns is the Ford F-150 SVT Raptor. It'll toss aside police cars like toys, especially at lower heat levels. Start a pursuit by ramming a police car. More police will spawn behind you, so do a 180 and drive into them for some head-on takedowns. Even more will spawn behind you as your heat level increases, so turn around and repeat until the achievement unlocks.
You can start an Ambush event as well, but these start on higher heat levels where it will be more difficult. Starting at heat 1 with a single car, you'll get plenty of easy takedowns quickly, and before things start escalating too fast.