For purpose of discussion, assume that due to bad luck, this asteroid has evaded the detection of all amateur and professional astronomers until about six months from impact. The asteroid is too large to deflect with humanity’s current spacefaring capabilities, and the general scientific consensus is that the impact will end all multicellular life on Earth.
What do those six months look like?
China and Russia would coordinate nuke salvos after having nerds calculate how to break it up to minimize loss of life/habitability. Depending on various factors other powers would collaborate.
The shotgun effect is hard to avoid, but with a ton of smart of people working on it I bet they could time the blasts to redirect the shrapnel to at least enough of a degree some parts of the planet survive. Triage might be needed.
A “fun” sideshow would be the religious right reacting to this. Evangelicals would swarm the holy land and possibly ignite violence.