Easy Duel Win:

First use Expellmarias until the orange ball is giant. When it is in the air heading for your opponent, use Diffindo before it hits so they cannot move away in time. Note: This may not always work.
Use Expelarmus (the shield) to deflect the attack. Immediately after that, start charging Flipendo. Shoot flipendo either when it is full or when the deflected attack is halfway there, whichever occurs first. This will either cause the Flipendo to hit or the deflected attack to hit. Note: Hold the aim sight to lock the Flipendo spell on the enemy.
When you are dueling, press L, cast the Knockaback Jinx twice and dodge the reflected spell. This will not work on Gargoyles, but is particularly useful against Draco Malfoy in Defense Against The Dark Arts or Slytherins.
After you use your Expelliarmus spell to deflect the Flipendo attack, before the attack hits your dueling partner, use your Diffindo spell on them. This will cause them to fall down and get blasted by the Expelliarmus you just repelled, winning you the point.
Whenever you are doing a Wizard duel, there is an easy way to defeat your opponent. Launch a Flipendo Spell at your opponent immediately after you launch his spell back at him. He will not be expecting it. This does not work when you are dueling with a Gargoyle.
When you are dueling with people (and not statues), go to the back of the room then turn around. When your opponent strikes, hit it back. Go very close to the blue line but not or past it. Hit it when it returns to you and it will more than likely hurt your opponent. Repeat this as many times as needed.
To defeat Malfoy and his gang easily in duels, let them shoot Flipendo at you. Next, use Expelliarmus to reflect it back. Keep playing this little "tennis" game until the magic ball is at the maximum size, then slowly move closer to the other person. This will make it more difficult, but you will confuse them. This works best on Goyle.
When Prefects come at you with Locomotmortis, hit them with Flipendo and they will stop in their tracks.
When a Prefect's Locomortomortus is about to hit you, use your Expelliarmus spell to repel it, then run away. You can also Flipendo the Prefect before he puts the spell on you. If you choose to do this, just quickly tap Flipendo. Do not wind up. They will say "Ow!" and give you enough time to run away.