Oda gave us the blueprints for many of his fights this chapter. Look at the wording. Big Mom attacked her
internal organs nerfing Ulti, while Nami finished her off
externally. She would have never won otherwise. Sound familiar?
This is exactly what happened in Dressrosa against Doflamingo, and I suspect what will happen with Kid + Law against Big Mom.
Law = Internal Attack to nerf Big Mom
Kid = External Attack to finish off