To win a duel I believe you need to be ready for anything. In this case I would say that a levitation spell, wingardium leviosa, would be an excellent spell to use in a duel. This spell would allow you to dangle your opponent, or their wand, in the air, thus preventing a spell from being cast in the proper direction (or at all).
![[Image: UK3atr7.png]](https://i.imgur.com/UK3atr7.png)