Stemming from DulleNL's above recommendation, I have found an improved method that significantly speeds up the process.
You will need two spells:
1. Levicorpus (Left and Right stick "Left")
2. Charged Stupify (Pull Right stick "Down", once it charges Push Right stick "Up"
All you need to do is cast Levicorpus to stop your opponent, close distance, then do charged up Stupify; at close range all three Stupify shots will land on the disabled target.
This is faster as the charged up Stupify does a lot of damage and counts as three hits (one for each of the three shots it generates). When the duel is over and the points counting begins, every hit of Levicorpus and every hit of Stupify (charged) counts as a Great, instead of a Good. 7-8 Greats and you have your 5 star rating.
Hope this speeds the process up for some, waiting for Levicorpus victims to come down takes 20-30 seconds every time, this takes 20-30 seconds per match.