Easy one. After you completed the story once, you should have unlocked it very quickly. Either way it is advisable to use multiple variations of spells on strong enemies in order to down them more quickly (because your spells have to "reload"). So use Petrificus totalus, Confringo, Expulso on your enemies and you're good to go.
Info: Breaking an enemy's protego with Expelliarmus and finishing him off with another spell also counts.