Do IVs matter in the story battles in hard mode?

please do no to spoil me anything about the game or future battles

i know that in hard mode almost every major battle have max iv pokemon and my team surely doesnt have perfect IVs (pretty sure not even have a single 31 IV) but they at least have beneficial natures
yet the battle with the gym leader in the jade tower is pretty tough for me…
do i really need to breed and “correct” my ivs and evs?
or do i simply just get good?