By overcoming his weakness? Lol. Or did you forget Sanji kicked Big Mom alongside Luffy punching her?
Did you forget she can shapeshift?? There's plenty of ways for Oda to write that fight.
Did you forget she can shapeshift?? There's plenty of ways for Oda to write that fight.
Oda already explained it, Sanji blocked BM's attack with his kick, because how strong BM's attack was and it didn't hurt her.
She is obviously gonna find out about his weakness, so why would she transform into a man? lol