Being forced to act irrationally, particularly through demands from strangers, is something most people will not tolerate much longer. My response to such a command takes the form of a no-nonsense statement while maintaining no-nonsense eye contact: "You can no more force me to put on a mask than I can force you to stop acting like a bully."
If the bully persists and demands I obey them or leave, as I turn to go I say: "You need to know that I have as much contempt for you wearing a mask as you do for me for not wearing one, you bully (or punk; whatever)."
At any hint of nearby Karen behavior, I start coughing immediately.
It works wonders!