Duelling is often depicted as a "tennis match" exchange of spells back and forth.

However, there is another approach which can be used, perhaps most suitable to a single powerful wizard fighting against a number of inferior enemies, where the duel consists of the powerful wizard casting a single very powerful spell and the enemies struggling to combat the effects of that single spell.

A depiction of this approach can be seen in the Crimes of Grindelwald movie. For all its faults, the depiction of Grindelwald using a single Protego Diabolica spell to utterly dominate 20 or so Aurors, and the ongoing struggle of his enemies against that spell as they fought to contain it, was an excellent scene and a very different approach to magical confrontation.

In a similar way, a Fiendfyre spell can be used in the same way, with the casting wizard using that single spell to control the arena, attack in multiple places at once, and also defend themselves at the same time.

What other approaches to "single spell duelling" can you imagine?

One approach that you typically do not see in HP magic would be a "summoning" type approach, where you use a spell to create/summon a magical being/construct (or a group of them) that fights for you, allowing the casting wizard to pursue their objective while letting that single spell perform the battle for them.

 

‘Enough of this rubbish!’ said Fudge, pulling out his own wand. ‘Dawlish! Shacklebolt! Take him!’

A streak of silver light flashed around the room; there was a bang like a gunshot and the floor trembled; a hand grabbed the scruff of Harry’s neck and forced him down on the floor as a second silver flash went off; several of the portraits yelled, Fawkes screeched and a cloud of dust filled the air. Coughing in the dust, Harry saw a dark figure fall to the ground with a crash in front of him; there was a shriek and a thud and somebody cried, ‘No!’; then there was the sound of breaking glass, frantically scuffling footsteps, a groan … and silence.

Harry struggled around to see who was half-strangling him and saw Professor McGonagall crouched beside him; she had forced both him and Marietta out of harm’s way. Dust was still floating gently down through the air on to them. Panting slightly, Harry saw a very tall figure moving towards them.

‘Are you all right?’ Dumbledore asked.

‘Yes!’ said Professor McGonagall, getting up and dragging Harry and Marietta with her.

The dust was clearing. The wreckage of the office loomed into view: Dumbledore’s desk had been overturned, all of the spindly tables had been knocked to the floor, their silver instruments in pieces. Fudge, Umbridge, Kingsley and Dawlish lay motionless on the floor. Fawkes the phoenix soared in wide circles above them, singing softly.

 

I liked the part of whichever Fantastic Beasts movie where Dumbledore just kept disapparating away from Credence's spells mid-speech and then just shut him down. What can you do when your opponent just disappears and reappears willy nilly?

Unless you can cast the counter charm.

 

I have never been 100% clear on whether this is a single spell or whether it is more of a traditional duel and it's just that Harry is disoriented / taken by surprise so unable to properly follow what is happening. There is a reference to a "second silver flash" so that could conceivably be a second spell.

But yes, one interpretation is that it is a single silver spell that whooshes here and there, impacting multiple people to knock them out rather than ending after hitting the first person.

You could take that idea and run with it, if you wanted to, and have this spell persist indefinitely, hovering around the caster and shooting off to take out any combatants the moment they appear, then returning to the caster but not dissipating.

 

In a way, Dumbledore does just that during the fight in the Atrium, although partly in reverse. The statues of the Wizard and the Witch are animated and used to block spells and protect Harry and impediment Bellatrix, while the statues of I think it was the goblin and the house-elf were sent to fetch/alert the Aurors (so I guess Dumbledore enchanted them to be able to talk since they also took the Floo to get out iirc). Here, Dumbledore deals himself with the mean threat and uses his constructs to pursue his other objectives.

Other than that and the other cases already mentioned, it's true that this isn't something we see much of in the series. Since we only see Dumbledore and Grindelwald accomplish such feats, I'm inclined to think that such a way of fighting would only be accessible to the extraordinary when fighting against trained fighter. An argument could be made that those in the competent Auror/Hogwart professor/Death Eater tier could use a single ongoing spell or a construct to hold off opponents that are much lower in skill than them such as students or untrained adult civilians.

 

Not exactly duelling, but Malfoy did something similar in a way in the finale of HBP, when he threw Instant Darkness Powder and grabbed his Hand of Glory. He used it to guide the Death Eaters to the next corridor unimpeded, but he likely could have pretty easily picked up Ron&co while they stumbled in the dark.

Another way for the 'leave a group to deal with something as I leave' approach, a Botany expert making a Devil Snare resistant to fire and casting an Engorgio on it (or giving it magical fertiliser) seems like something that would take a while for most wizards to get rid of.

You could also take a page from Hermione's book and leave them to deal with a giant instead of you.