It's an affirmative defense, so the standard of proof would be mere preponderance of evidence, i.e., a reasonable person would conclude, based on the facts asserted by the defense, that it's more likely the case than not. I think such things as her voice having been recorded saying, right before the shooting, "I'm going to taser you," while holding the Glock, and then her voice, after the shooting, expressing surprise, stating "I just shot him," would satisfy the standard.
My wife just shook her head and said" Women have no business being street cops. They can't handle the stress.". She had worked for a while as as dispatcher at our local PD and said she couldn't handle it. She's a real anomaly, a level-headed woman. She is the first to say the same.
