"On a high hippogriff" ~on a high horse (Arrogantly believing oneself superior to others)
"The quaffle's on your pitch" ~the ball's in your court (It's up to you to make the next decision)
"Poisonous toadstools don't change their spots" ~a leopard can't change its spots (one can't change basic aspects of their character, especially negative ones) ((this one's canon, yeah?)
"Yanking your wand" (joking around)
"Taste of your own potion" ~taste of your own medicine (something that you do to somebody else happens or will happen to you)