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 (along with proffered evidence), 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.
