When you get up to hogwarts grounds go down the steps and to your left you will see 3 big grey boxes. Go up to them and cast wingradium leviosa and the other 2 will join then stack 2 on each other then put one on the ground in front of the other 2 then climb up on to them then turn to your right and there is a triwizard sheild there.

To get your spells more powerful hold a for a few seconds and then release it and you wont haveto work as hard to destroy any creature.