Once he gets the sword, Harry adopts a variety of two-handed fighting styles: sword and knife (with the knife parrying), knife and wand (with the knife parrying again), and sword and wand (with a fluid mix). That last style is especially hard to combat, as the sword has enough reach that he can attack and defend with either hand. Combined with his athleticism allowing him to dodge many spells, he is a very difficult opponent for most wizards to handle—even Voldemort struggles to deal with the sheer amount of crap Harry can force him to deal with.