When dueling, use Levicorpus on your opponent. Then, while he or she is up in the air, get as close as you can to them. Charge Stupefy as much as possible (the light from your wand will blink two times separately). Cast the charged Stupefy on your opponent before he or she comes back down. If done correctly, this will take away six of the health "dots", which is enough to completely defeat some opponents. Use this as many times as desired against opponents with more health. This will not work as well in two player dueling because your human opponent can wriggle free pretty quickly after being hit by Levicorpus.
Harry Potter And The Half-Blood Prince
Easy victory in duels