Here is my stab at a solution (already posted at ffnet):
First Harry tells V. that Dementors are death, Patronuses work by not thinking about death and the true Patronus works by using a diferent mindstate which V. probably cannot attain (without specifics). Second Harry states that as long as Dementors are around every person including V have in each moment a small but finite probability to be kissed by one. Over an indefinite timeframe the aggregate probaility that V. is kissed approaches one. How this would interact with V’s Horkruxes is unclear but he may easily suffer a fate worse than death. Therfore he should keep Harry around at least until the dementors are dealt with.
Then he points out that given what he knows about the ambiguity of prophecies the prophecy V. heard has probably not clearly identified that Harry and not V. is the threat. Thus V. killing Harry might easily doom the world. This is especially likely as V. is not bound by the vow. Thus V. should keep Harry around to guard against his own mistakes and probably take a similar vow. He himself may offer more vows to further Vs goals in exchange for V. vowing to further Harry’s goals and so on. This should be beneficial for even a purely selfish V. who wants the world to survive.
In case V. is not convinced by his above offer of cooperation Harry uses the time they are talking to prepare for an attack on V. and the Death Eaters using partial transfiguration:
Thinking about venues for attack he first thinks about transfiguring an invisible nanoweapon such as a monofilament knife to decapitate the death eaters. Though he quickly realizes that that will not work since no known material including carbonanotubes is stiff enough to form an invisible blade of several meters length. Independently acting nanobots are out too, because he lacks time and knoledge to design one let alone test them for safety and efficiency. Then he realizes he does not need them, because partial transfiguration can do everything a nanobot could and even more.
He points his wand to a patch of skin on his leg and starts to transfigure the stratum corneum. An invisible bundle of carbonanotubes extends from his skin to the ground branches out to each death eater running up their robes and into their necks. (They do not feel this, since the bundle of tubes has a crossection of only 50 nm. Pain or touch receptors would not pick that up.) Another branch extends to the Dark Lord, but Harry does not dare touch him with his construct fearing the resonance. Instead he builds a small tower form the ground using carbonanotubes in a pattern resembling the Eiffel Tower extending right into the muzzle of his gun (Beneath the moonlight glints a tiny fragment of silver, a fraction of a line...). He seals the muzzle with a thin sheet of carbonanotubes and fills the barrel with nitroglycerine contained by a second thin sheet of carbonanotubes just before the bullet. All of this is very low volume and quickly transfigured.
If the Dark Lord refuses cooperation he snaps his fingers and immeadetly extends the tube in each of the death eaters neck to severe the brainstem from the spinal cord, the language center from the brain (to prevent wordless, wandless magic) and the neck from the body (black robes, falling). To make sure that everything is properly seperated he turns his entire construct (except for the part in Vs gun) into pressurised air (...blood spills out in litres,...). Now the Dark Lord either surenders or fires his gun. …and Harry screams a word: “rennervate” and points at Hermione to wake her up. Hermione stunns V. Even if V. fired he should not die immeadetly except if part of the gun passed through his brain. Hermione transfigures V. into a small stone to prevent him from dying and thus from coming back. Afterwards they transfigure the Death Eaters for eventual revival.
I wrote multiple redundant plans, because I genuinely think Harry should be able to convince V. to cooperate for purely selfish reasons. But even if V. is not only rational and selfish but “For the Evulz” Evil and thus refuses, the transfiguration attack should secure Harrys victory.
Here is my stab at a solution (already posted at ffnet):
First Harry tells V. that Dementors are death, Patronuses work by not thinking about death and the true Patronus works by using a diferent mindstate which V. probably cannot attain (without specifics). Second Harry states that as long as Dementors are around every person including V have in each moment a small but finite probability to be kissed by one. Over an indefinite timeframe the aggregate probaility that V. is kissed approaches one. How this would interact with V’s Horkruxes is unclear but he may easily suffer a fate worse than death. Therfore he should keep Harry around at least until the dementors are dealt with.
Then he points out that given what he knows about the ambiguity of prophecies the prophecy V. heard has probably not clearly identified that Harry and not V. is the threat. Thus V. killing Harry might easily doom the world. This is especially likely as V. is not bound by the vow. Thus V. should keep Harry around to guard against his own mistakes and probably take a similar vow. He himself may offer more vows to further Vs goals in exchange for V. vowing to further Harry’s goals and so on. This should be beneficial for even a purely selfish V. who wants the world to survive.
In case V. is not convinced by his above offer of cooperation Harry uses the time they are talking to prepare for an attack on V. and the Death Eaters using partial transfiguration: Thinking about venues for attack he first thinks about transfiguring an invisible nanoweapon such as a monofilament knife to decapitate the death eaters. Though he quickly realizes that that will not work since no known material including carbonanotubes is stiff enough to form an invisible blade of several meters length. Independently acting nanobots are out too, because he lacks time and knoledge to design one let alone test them for safety and efficiency. Then he realizes he does not need them, because partial transfiguration can do everything a nanobot could and even more.
He points his wand to a patch of skin on his leg and starts to transfigure the stratum corneum. An invisible bundle of carbonanotubes extends from his skin to the ground branches out to each death eater running up their robes and into their necks. (They do not feel this, since the bundle of tubes has a crossection of only 50 nm. Pain or touch receptors would not pick that up.) Another branch extends to the Dark Lord, but Harry does not dare touch him with his construct fearing the resonance. Instead he builds a small tower form the ground using carbonanotubes in a pattern resembling the Eiffel Tower extending right into the muzzle of his gun (Beneath the moonlight glints a tiny fragment of silver, a fraction of a line...). He seals the muzzle with a thin sheet of carbonanotubes and fills the barrel with nitroglycerine contained by a second thin sheet of carbonanotubes just before the bullet. All of this is very low volume and quickly transfigured.
If the Dark Lord refuses cooperation he snaps his fingers and immeadetly extends the tube in each of the death eaters neck to severe the brainstem from the spinal cord, the language center from the brain (to prevent wordless, wandless magic) and the neck from the body (black robes, falling). To make sure that everything is properly seperated he turns his entire construct (except for the part in Vs gun) into pressurised air (...blood spills out in litres,...). Now the Dark Lord either surenders or fires his gun. …and Harry screams a word: “rennervate” and points at Hermione to wake her up. Hermione stunns V. Even if V. fired he should not die immeadetly except if part of the gun passed through his brain. Hermione transfigures V. into a small stone to prevent him from dying and thus from coming back. Afterwards they transfigure the Death Eaters for eventual revival.
I wrote multiple redundant plans, because I genuinely think Harry should be able to convince V. to cooperate for purely selfish reasons. But even if V. is not only rational and selfish but “For the Evulz” Evil and thus refuses, the transfiguration attack should secure Harrys victory.