Instructions
Play through the game as you normally would. During Year 1, Chapter 4: The Restricted Section, you might run into Filch and Mrs. Norris when navigating the halls at night. If you do, press the "2" button on the Wiimote to cycle through the various spells and objects you have available to you. Keep pressing the "2" button until you reach the icon for Harry's invisibility cloak.
Press the "B" button to put on the invisibility cloak. The cloak allows you to walk right past Filch and Mrs. Norris without being seen.
Press the "C" key to switch your main character to Ron if you get caught by Filch as Harry. While playing Ron, shoot a spell at Mrs. Norris by pressing the "B" button. Filch will let go of Harry and you can switch back by pressing the "C" key again. You can either quickly put on your invisibility cloak or run quickly away from Filch and Mrs. Norris so he can't catch you again.