(EDIT: I thought about this some more, produced a better solution, submitted it as a review, and also posted it here)
Voldemort knows that Harry possesses an altered version of the Patronus spell, and something else that affects dementors, but not the spell’s nature. Harry can buy time by offering to explain how these work, by doing so, and negotiating for more names. Harry can truthfully say that learning certain things about the Patronus and about Dementors will have side-effects that Voldemort may not want, and that this means he needs to think about what to say. Voldemort will surely exert time pressure, but can only speed this up by so much if he wants to fulfill his goal of gaining all of Harry’s secret powers. He can also try to convince Voldemort to let him cast his modified patronus; this is very unlikely to work, but should be done anyways because seeming to not try any tricks would itself be suspicious.
A winning strategy should simultaneously disable all of the death eaters present and Voldemort himself. Voldemort will be disabled if their magics touch, especially if that touch can be sustained.
The main weapon at Harry’s disposal is partial transfiguration. I spoke first of buying time, because we also know that transfiguration takes time, although we don’t know all the details of how the time requirement is determined or where in the process there is first external evidence of a transfiguration in progress. Harry has already practiced transfiguring thin cross-sections through things, and used this power in anger twice (in Azkaban and against the troll), so he should know what he can do there, how long it will take, and when it will become visible.
Harry can only transfigure something that touches his wand. So he transfigures a narrow segment starting at his wand, passing through himself into the ground, and proceeding from there.
Since one turn of Harry’s time turner remains, if a plan ends with Harry recovering his items, then there may be an extra invisibility-cloaked, fully equipped copy of him nearby. While there is a charm that Voldemort could have cast which would detect if that were so, we don’t actually see him cast it. However, the resonance effect would reveal his presence; so instead, if Harry manages to time turn and escape, he should immediately contact and lend his cloak to Mad-Eye Moody.
If Harry can get a partial transfiguration off, there are a ton of possibilities. Harry has recently learned how, in a complex transfiguration, to control the order in which parts appear. So these options are not mutually exclusive; he can perform several or all of them at once.
There is a time turner nearby, in a pile of objects on the floor which no one is paying much attention to. He can modify the floor under it, flipping it over.
There are objects transfigured by Voldemort nearby (false teeth), in the same pile; if Harry’s transfiguration touches one of them, it could cause resonance. Many of the other objects in that pile will also be enchanted by Voldemort’s magic; transfiguring the entire surface underneath the heap would strengthen the effect. (This is important because Harry’s transfiguration can’t reach Voldemort himself, since Voldemort isn’t touching the ground).
There is a Hermione horcrux nearby, which activates by touch. What “touch” means in the context of partially-transfigured cross sections has not been nailed down, but if that does count, then Harry can cause the hermione horcrux to simultaneously touch all of the death eaters. Furthermore, he can cause it to not simultaneously touch himself, by transfiguring a bit of floor into a delay switch, and lifting up his foot after the transfiguration is complete.
There are weapons Harry can make with low volume and low mass. If the configuration is not too complex for the time available, he could try transfiguring a cross-section that passes through the brains of each of the death eaters.
Hermione is nearby, asleep. She will wake up if a little bit of her skin turns into acid.
Ideally, he would do all of these things as part of the same transfiguration; but that might be too much.
(EDIT: I thought about this some more, produced a better solution, submitted it as a review, and also posted it here)
Voldemort knows that Harry possesses an altered version of the Patronus spell, and something else that affects dementors, but not the spell’s nature. Harry can buy time by offering to explain how these work, by doing so, and negotiating for more names. Harry can truthfully say that learning certain things about the Patronus and about Dementors will have side-effects that Voldemort may not want, and that this means he needs to think about what to say. Voldemort will surely exert time pressure, but can only speed this up by so much if he wants to fulfill his goal of gaining all of Harry’s secret powers. He can also try to convince Voldemort to let him cast his modified patronus; this is very unlikely to work, but should be done anyways because seeming to not try any tricks would itself be suspicious.
A winning strategy should simultaneously disable all of the death eaters present and Voldemort himself. Voldemort will be disabled if their magics touch, especially if that touch can be sustained.
The main weapon at Harry’s disposal is partial transfiguration. I spoke first of buying time, because we also know that transfiguration takes time, although we don’t know all the details of how the time requirement is determined or where in the process there is first external evidence of a transfiguration in progress. Harry has already practiced transfiguring thin cross-sections through things, and used this power in anger twice (in Azkaban and against the troll), so he should know what he can do there, how long it will take, and when it will become visible.
Harry can only transfigure something that touches his wand. So he transfigures a narrow segment starting at his wand, passing through himself into the ground, and proceeding from there.
Since one turn of Harry’s time turner remains, if a plan ends with Harry recovering his items, then there may be an extra invisibility-cloaked, fully equipped copy of him nearby. While there is a charm that Voldemort could have cast which would detect if that were so, we don’t actually see him cast it. However, the resonance effect would reveal his presence; so instead, if Harry manages to time turn and escape, he should immediately contact and lend his cloak to Mad-Eye Moody.
If Harry can get a partial transfiguration off, there are a ton of possibilities. Harry has recently learned how, in a complex transfiguration, to control the order in which parts appear. So these options are not mutually exclusive; he can perform several or all of them at once.
There is a time turner nearby, in a pile of objects on the floor which no one is paying much attention to. He can modify the floor under it, flipping it over.
There are objects transfigured by Voldemort nearby (false teeth), in the same pile; if Harry’s transfiguration touches one of them, it could cause resonance. Many of the other objects in that pile will also be enchanted by Voldemort’s magic; transfiguring the entire surface underneath the heap would strengthen the effect. (This is important because Harry’s transfiguration can’t reach Voldemort himself, since Voldemort isn’t touching the ground).
There is a Hermione horcrux nearby, which activates by touch. What “touch” means in the context of partially-transfigured cross sections has not been nailed down, but if that does count, then Harry can cause the hermione horcrux to simultaneously touch all of the death eaters. Furthermore, he can cause it to not simultaneously touch himself, by transfiguring a bit of floor into a delay switch, and lifting up his foot after the transfiguration is complete.
There are weapons Harry can make with low volume and low mass. If the configuration is not too complex for the time available, he could try transfiguring a cross-section that passes through the brains of each of the death eaters.
Hermione is nearby, asleep. She will wake up if a little bit of her skin turns into acid.
Ideally, he would do all of these things as part of the same transfiguration; but that might be too much.
He doesn’t need to stall for time to transfigure. He could have already been doing it over the last two chapters.