Harry might not be able to transfigure air into solid objects, but he can still transfigure air into other gases, right?
Even if the Death Eaters’ masks had some kind of air filter enchantment on them, they likely don’t anymore:
We… we were not fighting in them, Master, with you gone… so I did not maintain their enchantments...
So perhaps Harry could partially transfigure the air around the semicircle of Death Eaters into some highly toxic invisible gas while leaving enough normal air between the gas and himself that it would be slow enough to propagate that he would have time to deal with Voldemort and get away and/or diluted enough that it wouldn’t incapacitate him?
Harry might not be able to transfigure air into solid objects, but he can still transfigure air into other gases, right?
Even if the Death Eaters’ masks had some kind of air filter enchantment on them, they likely don’t anymore:
So perhaps Harry could partially transfigure the air around the semicircle of Death Eaters into some highly toxic invisible gas while leaving enough normal air between the gas and himself that it would be slow enough to propagate that he would have time to deal with Voldemort and get away and/or diluted enough that it wouldn’t incapacitate him?