I posted a longer form of this as a review / solution. Here's a condensed version:
Partial Transfiguration works through a deep understanding of physics. It allows Harry to to create any physically valid state of the universe, as long as he can hold it in his mind.
What this means is that you don't need to Transfigure a gun in order to fire a bullet. You can just Transfigure a bullet in the state of having been fired.
This is what the ability to Transfigure any physically valid configuration really means. You don't need to make a bulky laser weapon. Just make a laser pulse: an arbitrary amount of high-energy photons, aimed in the right direction. Instead of a shaped explosive charge, make a shaped explosion. Instead of antimatter, make gamma rays. Instead of a black hole, dangerous to everybody near it, make a bunch of gravitons and aim them at your enemy.
So given all that, how should Harry kill his enemies?
Lasers are messy weapons. Even black robes are reflective in some wavelengths. Use too much energy and you'll get a fireball back in your face. Release the energy too quickly and it will create an explosion instead of steadily boiling away your target.
Kinetic energy is safer. Transfigure a set of diamond missiles-in-flight, one aimed at each Death Eaters and one also for Voldemort, who is conveniently floating behind them. Giving them a speed of, oh, 0.005c should do nicely. They should be as large as possible - in order to leave large holes - but, since the difficulty and length of Transfiguration scales with the size of the target form, they will be flat and thin: head-sized and a millimeter thick, lying on the ground in front of Harry until the moment when, Transfiguration completed, they instantaneously acquire the forward velocity (and some angular momentum) that will have them impacting the Death Eaters' masks a few microseconds later. The slight layer of air turned into plasma carried in front of them will serve as a nice bonus.
Transfiguring the ground in front of Harry, if possible, is the best solution. Lacking that, pieces of Harry's legs will serve. Since Transfiguration can change the size and mass of the subject, the resulting wounds need not be deep.
If Transfiguration scales with the diameter of the target form, rather than its volume, we will reluctantly use much smaller bullets. A thousand diamond squares, one centimeter across and a millimeter thick, will form a sheet 31 centimeters on a side: much smaller than a car battery. An average of 26 .40 caliber bullets per head should be sufficient to the task.
So... what stops the dark Lord from seeing Harry's wand move, then immediately putting up shields? If Harry doesn't need to move his wand to perform transfiguration then fat enough bullets will work. I don't know what a reasonable wizard reaction time is, but it's safe to assume that 0.05c bullets will be too fast to notice. But if Harry has to move, LV can get up shields in time I think.
The next question is, what are you transfiguring? You don't appear to be able to transfigure the vacuum, and it's been established that air cannot be transfigured.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 113.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
IMPORTANT -- From the end of chapter 113: