Imagine an alien race that's advanced enough to wipe out humanity. They are here and they want Earth. They know we are not gonna let it go easy 'cause they've studied us for decades.
Their motivation isn't relevant, but they need our planet for some reason. Their objective is to exterminate humanity but they can't harm our home - at least not beyond repair. This means that:
- Nukes are out of the picture;
- If possible, they will not harm any other species;
Assuming they have very advanced technology, from a strategic point of view, what would be the best way to eliminate us as quickly as possible?
Imagine you are the alien commander. What would your tactics be? Kill the electronics with EMP's to disrupt communications? Bombard major cities first to avoid blowback? Spread known diseases like the black plague to kill them from within?
The ideal answer must take into account the vastly widespread human population and the many nations with their own military capabilities, 'cause people will try to fight back - or hide.
I'm changing the original statement that all humans must die. I can settle for 99,99% - in a way that it's impossible for humanity to ever recover.