Harry may have fought a war and served fifty years as an auror, but for the last hundred years, or so, of his life, he never did anything more advanced than the occasional household magic. Since he got here to this world, he’s hardly done even that. While at the Dursleys, he only ever really did small exercises to stimulate early growth in his magic core, with the occasional bit of actual spell work (like breaking the Trace, getting Riddle’s wand to acknowledge him as its master, minor healing spells now and then after angering his aunt and uncle). All this means one thing: he needs to brush up on his duelling.