I think you underestimate just how far behind the technological, and energy, curves humans would be against any species capable of leaving their solar system in any meaningful way, doing it routinely and with considerable numbers. Having FTL capability would seem necessary but even without that they would be far, far more advanced then we are.

What can you do about it ... not much. Advising people to do more than the usual instinctual response, dig a hole and hide, would be like advising ants to stock up on seeds and double the guard when humans start building houses next door.

If it was to come to pass you could dig a hole and stock up on beans and bullets, if that sort of activity makes you feel better, but really ... if they are who they must be to have gotten here ... and they are anything but friendly and/or entertained by us ... we are screwed.

The energies and technologies needed for extra-solar exploration/exploitation make planetary bombardment with asteroids trivial. They can drop an asteroid the size of a building anywhere on earth and get the explosive force of a nuclear weapon, with none of the nasty nucleotides.

It is okay to dream of mounting a bold and determined resistance and hunkering down for them to die off from endemic diseases but the bottom line is that if they want us gone ... we are gone. No amount of grim determination, or waving our ineffectual fists, will change that.

The good news is that he more we learn about other solar systems the more we understand exactly how unremarkable our little system is. So far, every assumption about what makes our solar system and planet special has been shown to be a regular feature of other systems and planets. So far, last I checked, suitable stars, water, presence in a habitable zone, and presence of organic molecules were all shown to be present elsewhere. Given the feebleness of our technology and limited reach if we can detect those there are most certainly a whole lot more out there. All that is good. If there is nothing particularly special about our planet there would be little reason to invade. Why would anyone cross a good part of a galaxy to get to what is on their doorstep?