Instructions
Watch the animated sequence of the troll smashing the bathroom stalls. As soon as the animated scene is finished, begin hitting the troll with your wand. The basic Reducto curse will work for the troll, so just slide the stylus quickly from Harry to the troll to hit him.
After repeated successful hits from Harry's wand, the troll will drop his club. The club is broken, so repair it using the Repairo charm.
Elevate the repaired club into the air using the Leviosa charm.
Move back to give the troll room to move away from the broken bathroom stalls. He will follow you into the center of the room, and the club will drop on his head, triggering the end of scene animation.
Watch the animation, then talk to Hermione Granger to finish the area.