LEGO Batman

In the Riddler's episode, chapter 3 "Green Fingers", you build the Venus ride-on for the first time. You'll know the place when you come to a garden area and see a lot of silver trees. You have to use the Riddler (or Mad Hatter in free play) to mind-control a guy to get the last pieces.

When you get to this area, try not to kill anyone as you bust up the flowers and green trees to build Venus. Then climb aboard and chomp chomp.

If you don't manage to have 15 guys to eat, you have two choices: 1) try again (the number of "eats" carry over, so if you only got 10, then you only have 5 to go), or 2) eat your companion for the last few (it takes 8 chomps to score a kill -- a little time consuming, but probably less so than starting over).

I just want to add that you can also obtain this achievement by doing the Arkham Bonus level. You'll find a Venus plant to ride on there as well as a bunch of enemies to eat.

killing your partner takes forever. i suggest going back into her apartment then exiting back into the garden. this way gives you more cops to eat and it is way faster. also you use the X button to chomp with. hope this helps.