If it’s really massive, there is nothing anyone can do.
It would be 12 hours of endless media hype. “Stay tuned for the latest about the end of the world.” “Latest photos on CNN.” “Pope says poor affected most.”
There absolutely is stuff you can do in that 12-hour window to preapare for a CME hitting earth.
Most apparent is the task of removing the sensitive components out of your car and protecting them.
Seems like a no brainer.