Knowing basic magic in order to defend yourself in duels is important. There are 4 main ones that should be able to give you the basic ability to defend yourself from Melee attacks, to have minions attack an enemy, to send an enemy away from you, and to cause minor physical pain.

Revulsion Jinx. To perform this spell, simply point the wand at the target, and say "Relashio", (Re-LASH-ee-oh). This will cause the target to release whatever it us holding on to, thus it can stop a target from strangling you, to having an enemy drop a weapon such as their wand or knife.

Oppugno Jinx. To perform this spell, simply point the wand at the target and say "Oppugno", (Oh-PUG-noh). This will cause beings under your control to attack the victim, such as things you have transfigured, Inferius, people under your imperio curse, etc.

Knockback Jinx. To perform this spell, move the wand in the motion as in figure 1.1, while saying "Flipendo", (fli-PEN-do). This will send your target flying backwards, effectively stunning them temporarily, and giving you more time to work with to do what you need to do.

 Fig 1.1

Stinging Jinx. To perform this spell, simply point the want at the target and say, "Aculeus", (uh-kyoo-lee-uh-s). This will cause marks to appear on to your target, causing physical pain and leaving a branding. This can also be used to blind the target, or shut their throat from swelling.

These 4 spells are all basic spells that should be used to perform Dark Magic. None of these are technical curses, and such should be treated as regular spells, such as in what can guard them, and ect.