Cast the Spongify spell on the spongify symbol on the ground. Then make Harry run near the Spongify symbol so that the Troll tries to get Harry. Get the troll to bang his stick on the spongify symbol. Make sure Harry runs out the way so he doesnt get hit. The stick bounces up and hits the troll in the head and stuns him. Do this three or four times in a row. This will kill the troll.