We've already seen Kid and Law's strongest techniques in rooftop while Big Mom is holding back against the supernovas since the rooftop. She used her haki only against Page One, even when her life was endangered and she was being thrown to the ocean by Kid and Law, she still didn't use her haki against them. The only way for Kid and Law to have stronger techniques than they already have is through a power up, but even then Big Mom not using her full strength until they can get a power up is retarded as f*ck. Again, the fight itself is very artificial, power levels don't exist anymore.