You're definitely not thinking creatively enough. You don't need to hit the planet directly with anything, all you need to do is destabilize the orbit of all t

You're definitely not thinking creatively enough. You don't need to hit the planet directly with anything, all you need to do is destabilize the orbit of all the planets or even just the Earth itself. You could send a gravity wave for example, that would cause the Earth's near circular orbit to shift to a very eliptical orbit. The vast change in temperatures would kill all life on the planet, and then you could come and mine all the resources that you needed and leave.