Befriend both the duck and the horses and fight the wizard who swapped their sizes.
Give the duck a flamethrower into her beacon. Now, she looks like like a real dragon and can destroy the wizrad’s castle.
Tell the wizard you are a certified dragon-fighter, and he should hire you instead of getting rid of the dragon (duck) himself. Give him some side task (bring you water (without magic, as dragons are resiliant to magic)) to keep him budy.
Meanwhile the horses would invade the castle and eat all his books and equipment.
Now, the wizard has no castle, no grimoirs and no money (paid it to you to “fight” the duck). You won.
Befriend both the duck and the horses and fight the wizard who swapped their sizes.
Now, the wizard has no castle, no grimoirs and no money (paid it to you to “fight” the duck). You won.