"The Spiderwick Chronicles" video game is a loose adaptation of the film of the same name. When playing the game, the player eventually comes across a river troll guarding a bridge. It is not possible to defeat the troll at the first meeting, but the player can trick the river troll into falling asleep to get past him. After acquiring a specific weapon, the player can return to the troll and turn him to stone.
Play through the game until reaching the quest called "The River Troll." This is immediately after the quest where the player tames a griffin. Use the character "Simon" when facing the river troll.
Attack the monster with Simon's splattergun, which pushes the troll toward the sunlight.
Shoot the river troll with the splattergun until it is all the way into the sunlight. A brief cut scene will occur and the troll will turn to stone.
When meeting the troll before having acquired the splattergun, use the Will-o-the-Wisp to lure goblins to the river. The troll will go to sleep once he has eaten 10 goblins.