I don't know how they'll do it, but I've come to accept Law and Midd will do the impossible. I give the fight to be concluded in the next 2 chapters.
Next BM will use ACoC and Midd will likely deliver the final blow. Everytime he managed to hurt her was when Law sneaked Big Mom iirc. I'd be down with Law awakening FS to once get the better of Big Mom then Kid awakening ACoC and using the giant sword to at the very least severely hurt + KO BM, if not kill.