Spiderwick Chronicles

The easiest way to do this is to chose Simon and start the Sprites and Goblins match on Frontyard. Run to the left by the cellar doors and stay there within the circle. Now just charge up the gun and spray the Goblins. Staying in that far left side I only got the green Goblins spawning a couple at a time and didn't encounter any Bull Goblins. Shouldn't take no longer than 20 mins, easy 100 GS.

I found another solution for this achievement.

First you start the Sprites and Goblins on Caves.
Choose Mallory ans use her sword.

Stay at the middle of the level and wait for the Pondskater-fee ( the blue one for invulnerability) to appear. Grap it and slay the Goblins above you. Then you run back to the place where the Pondskater-fee reappear. No Goblin appear so long you stay on your place. Wait for the next Fee. Rinse and repeat.

The tactic of standing near the cellar door seems to work for some, and not for others. I tried it, and no matter where I stood, the Blue goblins were able to hit me with rocks. I've heard the same from other people. I suggest trying it, and if it doesn't work, try this solution:

Early in the match, I grabbed a batch of Distract Sprites. I stayed behind the mushroom ring w/Simon, holding "A" to power-up the spray gun (that tactic works well). Once the Blue buggers showed up at around 90 points, I ran out of the ring and cast off a Distract Sprite. EVERY goblin ran to it...and they didn't defend themselves when I attacked them! I was around 95 points when the effect wore off, so I quickly cast off another distraction, and bashed a Blue and a Red for 100. Achievement Unlocked, and it worked on my first try.

Hope that helps. It certainly ended my frustration.