Unlock the IED crafting in the Skill Tree.Step 2:
Craft IEDs using electronic parts and chemicals from the pawn shop.Step 3:
Go to a busy location, shoot some people and have the cops called on you. Once the cops arrive, they will drive in pairs, and park their cars near your position. Throw the IED between the cars, and explode it in order to explode both cars and kill 3+ cops to unlock the achievement.