-To easily win pretty much any fight or duel,cast Levi Corpus or Petrificus Totalus. Then just charge your Stupefy until it chimes twice, then release it. It will result in a one hit victory until later in the game, then it will require two. Note: This does not work on Fenrir Greyback.
-Use the following combo to easily defeat any enemy. Use Expelliarmus to knock them down, then charge up your Stupefy. While getting it charged, walk next to your dueling opponent and fire your charged Stupefy for a one hit kill. This trick can give you 5 stars for the challenge very easily.