The first thing to be realized here is that Zetsu are fodder.
They'll have to fight while using the most minimum chakra. This is quite possible.
Onoki can make many golems(which are Susanoo sized btw) to crush the Zetsus. The tech costs little or no chakra.
Gaara can use his typical sand wave and crush them by the droves. Onoki can lighten his sand so that he can maneuver more will less chakra cost and then harden the sand when he captures the Zetsu.
Mei can go to another corner of the battlefield and use Kirigakure no jutsu and simply take out the Zetsus one by one.
A has shown that his base armorless speed can easily murder Zetsu, he will deactivate armor and attack them with normal taijutsu and immense strength.
Tsunade will summon Katsuyu and have the slug continuously spit acid while she herself engages them in normal taijutsu fights.
Now the Zetsus have shown to be able to combine and get larger(like they did against KM Naruto)
In this case Onoki can fly around the battlefield using his heavy weight tech to paralyze any giant Zetsu formations.
Gaara can simply crush the giant Zetsus with sand.
Tsunade's regular strength is enough to crush giant Zetsus
Raikage will don his armor and use his lightning slash(which could tear off the Eight Tails horn) to hack down giant Zetsus
Mei will simply activate her acid mist technique
Katsuyu will spit more acid
These tactics will continue with Tsunade refilling them at intervals with parts of her Byakugou chakra.
Assuming a general rate of 500 dead Zetsus per minute(sand crushing, taijutsu killings of A and Tsunade, acid and acid mist melting, golem crushing), they should be finished in about 100 minutes. Less than two hours.
