Kuzan in the first place would have probably failed to ambush Law since BB is way more cunning, also since Law is YC1+ level at least at this point it is unsure Aokiji would have been actually able to win.
And of course Aokiji just managed to fight with old Garp because he was helped by half of BB crew and Garp was dragged down by the greenhorns he had to carry. BB, helped by half his crew, would have surely won if even Rayleigh thought that BB is stronger than him now.