Precisely, Ope Ope hax gets nerfed all the time because Oda made it too busted
It's not "nerfed", it's the Ope's natural limit in a world where Haki conquers all. Law already said that hax like shambles doesn't work if the opponent's Haki is too strong. He had to resort to the shockwaves of his awakening to physically harm Big Mom. Pure hax, like changing personality or removing hearts might not work at all against someone with Haki of Shanks' level.
You also didn’t answer my question on who will beat a gorosei. Law is a prime candidate as one of the only 3 remaining powerful D fighters in the alliance — Law, Dragon, Luffy. This is a fight that he is deeply connected to, the immortality surgery is also a bonus. That means Law will last up until the boss fight against Imu
Even if we say it will be Law, will it be due to the Ope fruit, or will he obtain ACoC to combine with his fruit?
I'm not saying EOS Law won't be busted, I'm just saying the Ope alone cannot do the job and that Ope + ACoC won't automatically make you unstoppable.
Because Law already showed the extend of the Ope's power and it would not be enough to beat Big Mom 1 on 1 at a flat surface which doesn't happen to be a floating island. A fresh Big Mom would probably mid diff Wano Law 1 on 1. Given even 2 on 1, she was still able to fight, given she immediately attacked Yamato when falling through Onigashima.