300 miles in diameter hit the Earth? The slate would be "wiped clean", so to speak. Scientists think it may have happened 4 billion years ago.
If we had a few days/hours advance warning, all that could be done would be to get your affairs in order, vis-à-vis with personal relationships, etc.. When it became obvious that it wouldn't be a near-miss, you might consider cashing in your life insurance policies as well and throwing a party to "end all parties", i.e., one that would never be topped. I am sure beforehand it would spawn all sorts of doomsday "gallows humor" as we attempted to cope with the thought of the finality of this event.
Kind of spooky, but it is what it is. "Nature bats last."