In a duell it can be good to now spells that´s not killing but still are cool. In other words spells that can show you skills. Usefull spells in a duell is:

expelliarmus=make the one you duell against´s wand be thrown away

stupify=make the one you duell against get thrown backwards

diminuendo=make the one you duell against smaller (if you cast this spell, make the normal size again by casting the spell engorgio after the duell

You can also do the spell imobulus witch will make everything go in slomotion or you can use petrificus totallus witch will make the one you duell against frozen as ice.