Spiderwick Chronicles

The first time you meet the River Troll you'll need to feed him 10 goblins by getting them close to him, later on return to him while palying as Simon, carefully get close to the edge and squirt him over and over with your modified water pistol, eventually he'll back up into the sunlight and turn to stone.

as Simon you will meet the Troll (that looks suspiciously like a crocodile cross with a godzilla) in the river will ask you to feed him 5 goblins the first time then 10 the second, after that you will pass and get on with your mission, as you come back he will ask for another 10 goblins, well this time just shoot him with your squirt gun you'll need to keep doing this and he will keep moving backwards eventually he will turn to stone and that's when this achievement will unlock