I may have an idea how kid will be able to bebt big mum.
we know kid is lacking some offense right now, compared to Kaido and Luffy. Still his defense and stamina seem to be above Luffy. Maybe kid can use that.
but first one step back in story. Big mum mostly hurt and raged was back then through… Nami. Or better said, through Zeus. It seemed like he could do mostly damage to her, but how? Thunder alone or weather shouldn’t do it. I think it is because Zeus is part of big mums soul. Maybe hurting her in the most efficient way is to let her soul clash together. When to pieces of her soul are hurting each other, they can’t block each other, cause it is kinda a self-hurting.
keeping that in mind, maybe kid is able with his stamina and defense use his metal arms as cages for prometheus and Hera. Covering them in CoA and pushing them with every punch on big mum, forcing Hera and Prometheus to let free some thunder and fire, which then result in power ups for the direct hits from kid. So kid would be able to force big mums soul to clash each other with every hit, which negotiates her defense!