60 seconds
If Harry loses he will die, so he could as well start making plans that resulted in his death unless he wins. Which primarily meant screw the rules of transfiguration - he'll fixes that with the Stone if he wins. That was the time for reckless plans, and as many of them as possible.
He needed to think quickly, and quite probably tolerate a lot of pain. Since he needed some way to transfigure things, he nonchalantly pressed the wand against his thigh OR(for you see, harry knew that it was more probable that at least one of these things would work) used adsorbents on the end of the wand to make a string of atoms going through his arm OR used the skin of his hands for the same purpose OR used the tip of the wand itself to make a string of atoms going through his arm, focused on a small trail of atoms leading from his skin to the endothelium of this superficial femoral artery, and transfigured it to a mix of amphetamine and morphine. That wasn't as good as having the powers of a troll and a unicorn, but that was something.
50 seconds
Now that he had a little bit of superpowers, Harry needed super protection - against bullets and other quickly incoming foreign substances. He was transfiguring his skull and sternum into steel. If Voldemort fires his gun, this will protect Harry from immediately lethal wounds, and hopefully amphetamine with morphine will help him to continue fighting if he's wounded elsewhere.
30 seconds
Harry focused on a trail of atoms leading from his leg through the ground to the tombstones behind the Death Eaters, ads started transfiguring them to Iodine-131, a powerful source of beta particles, hoping to reach levels far above lethal. Unfortunately, this will not kill anyone fast enough to be useful in the battle, but it will lead to the acute radiation syndrome and the subsequent disorientation. Of course this will hurt Harry too, and if he doesn't treat his body with transfiguration and the Stone quickly enough, he will die too. However, his brain is now protected by steel, so despite the radiation poisoning, he won't dizzy and disoriented - and he will fight. There was no reason for the Death Eaters to even know what radiation was, much less have protection against it, and it's completely non-magical, so hopefully they won't notice anything before it's too late. Voldemort was very much worried about nuclear weapons, and Harry could only hope he wasn't paranoid enough to invent and keep anti-radiation charms all the time. Hermione's body will probably take care of the radiation poisoning itself. Now he had approximately 15 minutes to win.
0 seconds
"Time'sss up, Harry" Voldemort hissed.
"I am ready, teacher, I know what the power isss that you know not." Harry replied.
Harry quickly ran over what he could say. There was the secret of true patronus, idea that dementors are death. Way to control them with expectations. Magical genetics. Simple solution to mirror's runes (who would have thought that ancient secrets were protected by cryptography used by 5 year old children). Idea that the world is already in danger, as scientists have known for ages(from things like supernovae) and it would take some time before QQ would learn about all of them, thus putting the world in danger. And there was partial transfiguration, but he should save it as a last resort, since he needed to use it. He would just try to get as much time as possible by talking about these things-even if voldemort would cut him off abruptly on one of them, he would just pick another.
"Ssspeak then, and sssave a friend" Voldemort gestured benevolently.
"True ssecret of Patronuss. Life eater is death. Patronuss is like mind barrier; think about ssomething else and hope death ignoress you. Iss why you and I cannot cast it. We cannot fool oursselves into forgetting about reality when it is inconvenient.
True patronuss is not coward's trick. True patronus thought is belief that we can conquer death, that immortality is posssible. True patronuss is life. Dumbledore could never casst it, but I think you could, teacher. We could defeat death together, you and I."
"Of course! But I decline your offer. I will defeat death mysself. Letting you live iss too risky. Which of your loved oness sshall I sspare?"
Transfiguring while talking was hard, but he kept doing it. He tried to wake Hermione up by transfiguring a small trail of atoms leading to her carotid arteries to epinephrine, wishing to also trigger fight or flight response. If that succeeded, with her invincibility to physical damage, immense speed, and Voldemort's order to Death Eaters to not hurt her no matter what, she could incapacitate them all alone. Unfortunately, no matter how hard he tried, his spells couldn't penetrate her new protection, and she remained asleep.
So Harry went to yet another plan, and started transfiguring a carbon nanotube leading to his pouch. When the time comes, he will command this tube to contract, thus bringing him his pouch.
-20 seconds
"My mother," Harry said. "I do not believe prophecy can be so easily averted. I think it is fixed, just as you cannot alter the past with a time turner. The future can affect the past. Have you heard of Comed-Tea? It makes you want to drink it just before something unbelievable happens. Foolish wizards laugh, but it proves the future can affect the present. Ssuggests prophecy cannot be averted entirely. If true, killing me will channel the prophecy in ssome unknown direction. It would be safer to work with me. I do not wish to end the world. You have already ressurrected girl-child and bound me to vow. You know me well. I am ssafest posssible vehicle of prophecy."
"I know all thiss. It is not a ssecret worth a life. But if prophecy cannot be averted, we are doomed in any case. Then my actionss do not matter. So I musst assume it can be avoided. Prophecy referss to you. If you are dead beyond return, it cannot happen."
"Are you truly sso confident, even after Godric's Hollow? Let me help interpret prophecy. You are wisser than me, teacher, but I sstill might see ssomething you have not. Hass happened before. I am bound by vow, so telling me is ssafe. What if killing me bringss greater desstruction? Cannot know what kinds of ghosst I will leave, after cassting my Partonuss. Can you be ssure prophecy cannot act through another? Can you be ssure I won't get resurrected through your horcruxes? Magic seems to think we are the same thing."
Voldemort paused. He was actually thinking about it. Harry felt a small note of hope... "No. Too risky. Enough pleading for your life, boy. Do you have any more ssecrets for me before you die?"
-40 seconds
Harry had really been hoping it wouldn't come to this last, desperate plan. But some chance of success was better than nothing.
He finished the partial transfiguration he'd been preparing as he'd spoken with Voldemort. It would have been easy if he could impose a Form upon the air. It was just another piece of the quantum waveform... But in this most crucial moment, despite all his efforts, the air wouldn't change. Raising the stakes didn't mean you could do the impossible.
So he'd gone with the backup plan. An ever-so-thin line of sulfuric acid, down the back of his leg, across the outer layer of skin. He'd worried about the pain, but compared to the pain of losing Professor Quirrel and then Dumbledore, it was barely noticeable. Then beneath the earth, acid carefully snaking to 36 different Death Eaters. Then up their long black robes, all the way up to their masks. The lines of acid Harry was drawing were so thin, just a few atoms wide, the whole transfiguration was fast and nearly effortless, even for a first-year at Hogwarts.
And then...the final step, there was no going back now, even if Harry miraculously survived he would have to live with this moment forever...
"Yess, there are more ssecretss, teacher." Harry finished the partial transfiguration. His deadly lines of acid burned through the masks and skulls and brains of the 36 death eaters. Some of them at least. They screamed and grabbed their masks as if trying to grab their faces. Others apparently had wards, and they fired their spells right when the others screamed. Voldemort fired his gun, and wordlessly sent the Killing Curse into Harry, who didn't wait, and dodged even before the fire was open. But just a moment before that, there was other scream, much louder and higher pitched than those of the Death Eaters: Hermione screamed, as the final line of acid hit her arm. Harry figured that this won't kill or even hurt her in the long term, but surely will wake her up, since the original plan didn't work. All the remaining Death Eaters missed, which was unlikely in any other situation, but radiation poisoning did its job, as well as the sudden distraction. Even Voldemort had flinched, his aim had to be hastily retaken and Harry was already moving, so the shot went to the left, hitting Harry's shoulder, losing some momentum, and ricocheting to his head...
...But not through the dead center. That hurt like hell, but morphine, amphetamine, and the steel skull did their job. The bullet bounced, and continued casting his spell. Voldemort's still-hasty aim, the glare of the incoming hex which he still had not dodged because moving would impair his aim when there might still be more secrets that Harry could use to turn the tables further if given the chance, he had said there were more secrets, the continuing scream and the smaller target hidden behind the body of his paralyzed follower prevented that shot from being quite perfectly placed.
And Harry finished his incantation, with Hermione's scream masking the slight difference in pronounciation: "Stuporfy! Ma-ha-su!" Even if Voldemort dodges Stuporfy, Ma-ha-su will probably still hit him. Harry's wand was ripped from his hand, but the red bolt was already on its way. His former teacher was left with barely a fraction of a second to deal with the red bolt of magic heading towards him, his mind pressured by the sudden turn of events, the scream, the wonder of whether that second shot hit or not - and given that timespan, that context, he did not apparate away, he did not fly full tilt away from the attack, he did not stop it in mid-flight way he stopped Bahry's stunner in Azkaban, but dodged its trajectory in the first way he could think of, which was by dint of the recency effect the last way he had done so, the way that by coincidence would have let him resume firing precisely the soonest...
...if the stuporfy hadn't responded by changing direction to cross the negligible distance that Voldemort had leaned out of the way and struck him.
Harry's head exploded with pain as the magical resonance rebounded on him. But Voldemort was screaming too, clutching at his head and screaming. Through blurry tear-filled eyes, Harry saw the Dark Lord vanish, and a green snake fall to the ground.
At the same moment, General Chaos made his tube leading to the pouch contract, and without waiting for this to be over, shouted: "Hermione, grab the stone near you, or Voldemort will take it," hoping that she would react, and be confused later. At the same time, He made a gesture to his pouch to him his broom, and then the cloak. Zigzagging on the broom, and putting the Cloak on at the same time, he grabbed Voldemort's wand, and then: "Prismatis!" - he made a shield around the snake, restricting its ability to move or turn back to Voldemort. This won't stop the Dark Lord, but would win some time. He grabbed Hermione holding the Stone, and escaped from the anti-time-turner charms. Then it was the time to go back, and call for help. No one in Hogwarts can help him, but Moody and Amelia Bones could probably help too.