Easiest done on the Heroes Side, Episode 1 Chapter 4: A Poisonous Appointment.
Load up this level in Freeplay and you will see a large patch of dirt on the left side of the screen with a bunch of jumping seeds on it. Build all of these and you will have yourself a large garden of carrots.
Change to Batman with his explosive suit, plant a bomb in the center of the carrots, get to a safe distance and hold the B button to detonate them. This will take out at least 12 carrots and get you the achievement.
This same screen is also a great place to get: