See, if any Black Keys pin his shadow, Voldemort can nullify their effects by turning them into snakes with his transfiguration. He's able to do this with items of an inherently magical nature, as he did so to a fire-rope that Dumbledore sent in his direction with magic. And again, he can also use Confringo to cause them to explode before Kirei throws them.
Kirei's AM will mess him up if it hits (assuming Voldemort's mind resistance doesn't soften the effects at all), and any attack that connects with Voldemort physically will just about kill him in one hit. but Voldemort is a lot more prepared to defend himself in ranged combat than Kirei is. Protego has protected him (and numerous other HP wizards) from spells that could obliterate stone statues bigger than humans, and far less-skilled wizards such as Dolores Umbridge have used it to block mundane projectiles like arrows and shards of glass. In Order of the Phoenix, Voldemort himself was almost impossible to hit with basic attack spells during his duel with Dumbledore, mostly because he was swatting projectiles down with silver shields and Protego, and because he was abusing his ability to teleport.
His own ranged game is also a lot scarier than Kirei's. Even without curses, he has mind manip which Kirei doesn't have resistance to, telekinesis which can be used to shove him off his feet, create shockwaves and/or hurl pieces of the environment at him, the aforementioned Confringo which can cause explosions, and Fiendfyre which is essentially a homing attack that will burn Kirei into ash the moment it touches him. (And can potentially harm souls, as it's one of the only things in HP which can destroy a Horcrux)
Also, again, Legilimency allows him to actively read minds. And much like Snape, he does this a lot during combat, to the point that it borders on precognition. Kirei won't have the element of surprise here, and Voldemort will have the advantage of knowing what all of his attacks can do.
