How did Harry beat the troll in Harry Potter and the Philosopher's Stone?

Answer 1

Harry was under the protection spell his mother offered when she sacrificed herself for him. This lead to an "old magic" being bound between Harry and Voldemort which meant that it was impossible for Voldemort to touch him in anyway (physical, or magical). This lead to both Voldemort's first and second defeat in the first book.

Professor Quirrell (while Voldermort being bound to his soul) tried to choke Harry but once Harry touched him with his bare hands, the part Harry touched began to burn severely. Then as Quirrell tried for a second time to kill him to obtain the Philosopher's (Sorcerer's) Stone, Harry touched his face and his entire body began to burn.

This duel between Harry and Voldemort (Quirrell) almost killed Harry, but Dumbledore came just in time to save him. Voldemort's soul then left the remains of Quirrell and departed for wherever he went until resurfacing three years later during the fourth book.

Answer 2

Harry Potter didn't kill Lord Voldemort in Harry Potter and the Philosopher's Stone.

What was left of Voldemort was sharing Professor Quirrell's body. Because of this, Quirrell couldn't touch Harry without suffering severe burns and blisters. Harry used this to his advantage and Quirrell died as a result of his injuries, leaving Voldemort to flee.

Answer 3

Ron Weasley began distracting the troll by making noise and throwing things at it while Harry Potter attempted to get Hermione Granger to safety. When the troll got confused it started going towards Ron, who had no way to escape, so Harry ran forward and jumped onto the trolls back, hanging onto its neck. When he jumped, Harry's wand was in his hand and it went up the trolls nose. Ron then used Wingardium Leviosa to levitate the trolls club and then drop it on his head, knocking it out.