In the "Dumbledore's Army" level, use Hermione's Crookshank skill to dig up Dean who is trapped on the right of the screen. Once he is unburied, pick up his wand and hand it back to him. He will use this to dislodge ice which you can use Wingardium Leviosa on to build a bridge.
After building the bridge, cross over it and walk to the left of the screen. There you will find a quibbler dispenser. Stand on the orange glowing spot and press the circle command and this will dispense the paper. Do this 25 times for the trophy.