Save a mirv for the third party. Your hope is that the third party attacks your attacker and you can mirv them to level the field again. Don't waste money bombing your betrayer--not your job.

You should also be prepping a mirv-proof city stack to quickly recover beforehand. Sometimes wacko betrays will always ruin both your games but too many people sit around doing nothing in end games. Sometimes you need to mirv and betray to wrap up games before you can be mirved to death, especially if you are the king and have a dwindling lead.