I think that since this story is Harry Potter and the Methods of rationality, its going to be Harry who saves the day. But if this wasn't a story...

Harry is in a totally impossible position (baring some experimental transfiguration). The best hope lies with McGonagall. She's been seen going off scrip before and if she realises the magnitude of the threat Volde now poses (perhaps Harry could impress this upon her, and point out that Volde has recently resolved to stop wasting time and just take over the world) she might realise that the only way left to stop Volde is to go totally off-script, and seek help from the Muggles.

While a gun can't hurt a shielded wizard, the shielding only has so much strength. Bring in the Muggle army with overwhelming numbers, heavy weapons, helicopter gunships and so forth. Wizards have access to apperation, dissilusionment etc which might still allow them to win against a pure muggle army, but a combine force of Muggles and Wizards would also have access to anti-apperation jinxes and so forth.

Admittedly, this ends centuries of secrecy. Therefore, the second option she has is to go to the Wizard UN or whatever their equivalent is, and explain that without reinforcements she will be forced to go to the Muggles for help.