An alien wants to destroy Earth, what would you do to convince them otherwise?

Don't bother. We'll destroy ourselves soon enough.

I once had a dream that I met God, who had decided that making the universe was a mistake and was, as I found him on the beach, beginning a ritual that would delete the universe.

I objected to this, telling him that while there were many negatives in the world (the important part to me), people were also capable of good. (I was younger and more optimistic then.) I guess I was convincing, because he pondered, then told me he would give me one year and Superman style powers to show the good of which humanity is capable. At the end of the year he would evaluate my actions and decide whether to finish his ritual.

I performed heroic acts throughout the year, but not enough to impress him, apparently. He told me at the end that he admired my actions but had decided to finish the ritual, and again I objected. We ended up fighting. I don't remember how the dream ended after that, but I woke up, so I guess one could argue that he did succeed at deleting that universe.

So I guess in your hypothetical situation, perhaps I would ask for superpowers.

"Just destroy the humans."