It's a titanic battle between two armies of immense scale, yet somehow in all the chaos, Major Hero and Ultimate Villan find each other and charge across the battlefield towards one another. The Hero is suddenly granted a superhuman ability to defeat opponents in one to two swings of his sword, usually without even looking at them, keeping his eyes locked on Ultimate Villain the entire time.
Generally the battle between these two progresses completely unmolested, though the war rages on around them. In some cases the two armies will even stop fighting and watch their heroes duel.
Usually, this one skirmish determines the entire course of the battle. The defeated party's army is suddenly routed and driven back or slaughtered. In some cases there is a You Killed My Father element as well ("That's for X, you Y...")
Another possible (non-violent) version of this is The One Big Dance, where the Charming Prince and the Beatiful Damsel join a formal dance floor packed with people, yet they end up as the only couple on the floor due to their otherworldly dancing skills.