Well, biological warfare is the only for-sure method. Good problem - how to preserve a small group of the race?
Well, they could just try to infect as many places as they could with exception to either parts of Africa, Antarctica, Greenland, or other places with little to no access to advanced weaponry. Whoever's left over after the pandemic has subsided can be test subjects.
The other races would have more problems with destroying humanity. If they could fire upon us from space, out of our missile range, they would have a rather large advantage. It would take time to develop the technology necessary to fire missiles into space. If they had enough ships, they could destroy most of civilization before that technology had a chance to be born.
Whether humanity will destroy the world or not so that neither race will be able to inhabit the planet is unclear. I don't know enough about humans to say.
If an alien race had to enter our atmosphere to wage war, they wouldn't be likely to succeed. They could perhaps make progress if they knew how to create wormholes wherever they wanted, but without a steady supply of soldiers their assault would be crippled very quickly. We would sustain considerable damage, but we would be 'victorious'.