Spiderwick Chronicles

When you kill a goblin you get teeth from them.
There are three different types of goblins and they give you different amounts of teeth.

Green goblins = 1 tooth
Red goblins = 2 teeth
Grey goblins = 3 teeth

When you have 250 goblin teeth you unlock this achievement. I got this in chapter 4

Don't worry to much about this achievement as you play through the game.

For the 1st six Chapters, just kill goblins as you run into them while you're going about your other business. You'll get the following amount of teeth from each type of goblin:

Green - 1 tooth
Red - 2 teeth
Blue - 3 teeth

Don't panic as you approach the end of the game. During Chapter 7 there is a huge goblin battle in the mansion where all of the children are wreaking vengeance on the goblins. Wreak some of your own while running around the 1st floor rooms of the mansion scooping up dozens of goblin teeth. I picked up around 200 just during this battle.

If you still haven't reached 500 by the time you complete Chapter 7, never fear. The Forest Road and the Deep Forest are lousy with goblins. All you need to do is go kill a few of them as you finish up the Field Guide.