Read Harry Potter and the Methods of Rationality (HPMOR) before this.
This was written as an answer to the author's challenge at the end of chapter 113 in HPMOR. Then I realised that it not just solved the immediate problem, but many others, and could be considered a complete ending for HPMOR.
This is not the best idea I had, but it is the most fun. Short, if not sweet.
Harry was facing an enemy as smart as him, who had carefully prepared for exactly this eventuality. But maybe Lord Voldemort would not think that a Tom Riddle without a Horcrux would have prepared a self-destruct.
It was time to lose.
Harry could not see a reliable way to live. He looked at the death eaters with defeat in his eyes. They were largely near the altar where Voldemort had been when he called them. Harry dropped his eyes in sadness, down to his far off pouch.
He lifted his eyes slightly, not all the way to where Voldemort was flying above the altar. He dropped his wand in defeat, and spoke sadly in Parselmouth.
"I can do partial transformation. I can imagine how I could use that to destroy the world. I could describe that to you if you wish."
"That may be worth my time. Proceed."
"It would take some time. I could transform part of the earth into U235. A subcritical mass, of course. Then I would transform another subcritical mass, not quite close enough to be critical. I could continue this for a long time building up a barely stable state. I assume you understand what happens when into close proximity come" Harry dropped his eyes down to his pouch, and spoke directly to it, still in Parseltongue, "two subcritical masses of U235."
When Harry had stocked his pouch, he had foreseen the need for self-destruction. He had transfigured two barely subcritical masses of U235, carefully surrounded with lead for the time he handled them. He had done this outside, way away from any ferrous objects, as he had also made a strong magnetic monopole on each. North on one, South on the other. Of course he had put the first into the pouch well before he started creating the second.
Now the two lumps came out of the moleskin pouch, in close proximity. The monopoles were strongly attracted, and the lumps raced towards each other, squishing the lead shielding. A hundredth of a second later a chain reaction started and in less than a millisecond overcame the magnetic attraction, driving the masses apart before much of an explosion had been able to build.
As nuclear explosions went, it was tiny. But for the death eaters standing nearby, it was still very effective. The nearest dozen were shredded, and everyone else on the field was at the minimum knocked out. Hermione's body was also shredded, thoroughly waking her up.
Thirty seconds later there was silence, broken only by the sound of Hermione's body stitching itself together. She stood up and surveyed the scene. Her last memory was of dying in Harry's arms. Now she was surrounded by dead and dying death eaters. It didn't make sense, so Harry Potter must have been involved. Sure enough, he there he was lying unconscious naked apart from his glasses near what appeared to be a stunned Lord Voldemort.
For twenty seconds she just looked, thinking carefully. Then she went looking for a wand.