An easy way to win almost any fight or duel is to 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 defeat any enemy. Use Expelliarmus to knock them down. Charge up your Stupefy. While getting it charged, walk directly next to your dueling opponent. Fire your charged Stupefy for a one hit kil. This trick can give you 5 stars for the challenge very quickly and easily.