(The title is misleading.)
I don't see how you can successfully resist an invasion without a centralized entity to coordinate things on a high level.
This is the argument used every time the topic of war is raised in a conversation that involved talks of anarchism and ending the State, and it did not fail to show up again in a conversation about Russia's invasion of Ukraine now.
Turns out there is a simple answer: if there was no State there would be no invasion because if you assume Ukrainian people wouldn't be able to organize a defense then you much more have to assume that the Russian people won't organize an attack.
The answer is unsatisfactory because there may be a Russian state organizing the attack while there is no Ukrainian State to organize the defense (because somehow the Ukrainian libertarians succeeded in ending the State just inside the borders of Ukraine). In this case it may be that the Russian State will occupy Ukraine and now the Ukrainian people will have to pay taxes and submit to psychopath politicians again, and Ukrainian libertarians will have another State to fight against.
The nature of the State
This situation, if it ever happened, would showcase again the nature of the State, which is, as described by Franz Oppenheimer, the apparatus formed by a group that conquered the another group. In this case the Russian high politicians and military conquered the people of Ukraine -- just like they had conquered the Russian peoples (or taken the control of the Russian government from others that conquered these peoples before).
What has changed?
If you compare the situation of Ukrainian people before the Ukrainian State ended and after the Russia dominated, has it worsen significantly? No. Maybe it is a little worse because the Russian State is worse than the Ukrainian State, but it could have been better if Ukraine had been conquered by some other country (could also have been worse).
What is to be done?
There is no real conclusion, i.e. I don't know what to do about Russia vs Ukraine. In this specific case maybe it makes sense to join the Ukraine government to defend against Russia -- if you think the Ukrainian government is so much better than the Russian. But to what point? I have no idea. The fight against the State will have to continue in any case.
Not necessarily
For the purposes of the reasoning above we granted that the Russian State would successfully invade and conquer Stateless Ukraine, but that is not certain. Many people have imagined ways in which a stateless society could fight back an organized army, and these ideas are not more absurd than some of the things we see in the real State vs State war.