IMO, your scenario is only equivalent had the 40 pounds bigger woman (and martial arts trainer) *FOLLOWED* the 12 years younger kid into the dark alley, muttering "!@#$% kid..!", and then later exited the alley, with the kid lie dying of a gunshot wound to the heart, her hair in disarray, suffering a few bruises, claiming self-defense to the police.
IMO, those details are important and telling.
Thank you for delineating the self-defense premise:
1) He or she reasonably believes that such force is necessary to prevent imminent death or great bodily harm to himself or herself
IMO, if there's no evidence of potentially "great bodily harm" to GZ, GZ's self-defense/his own personal account will fail to convince the jurors. He definitely suffered from minor, superficial wounds as testified by medical examiner and TM had zero, absolutely no DNA of GZ on his hands or body. The kind of flesh and blood DNA as a result of 25 times of pummeling will not be all washed away by rain, IMO. Therefore, in the absence of evidence and with only the inconsistent testimony of GZ, it is not at all convincing that deadly force was justified as per the self-defense law. IMHO.