I honestly found this much easier by constantly moving left and right and casting spells. When you start immediately start moving left and right and cast levicorpus repeatedly until it hits, then switch to another spell and hit him with that until he drops. Do this over and over, but make sure you use all the spells at least once. So levicorpus, incendio, levicorpus, stupefy, levicorpus, expellarimus, levicorpus, immbolus. I was able to get 5 stars with a score of 55 in the accuracy section.