Instructions
Use the "Leviosa" spell on any Lego blocks you see in front of the troll. These blocks will levitate and hit the troll in the head.
Use the "Leviosa" spell on any Lego blocks the troll sends at you with his club. Hit him in the head twice with these blocks, and he will become dazed.
Use the "Leviosa" spell on the troll's club while it is dazed. Move the club above the troll's head, and the club will fall upon the troll, defeating him.