@Cucoo
Revised. I have covered all bases. The doppelganger will now be unable to react.
Yukari goes back in time to before the battle starts, or even before they met the doppelganger and tells Nue to make them unidentifiable. She also tells Kiene to alter the doppelganger's history, preventing him from retaliating. She is not directly affecting the doppelganger itself, only its history. The doppelganger will not be able to counter this because he doesn't even know who he's looking for now, and even then, he won't. Fast forward a couple days, and we're in the present, right before the battle.
Before the battle, Hina would remove all of Remilia's bad luck, making her capable of performing fate manipulation flawlessly, Remilia would alter fate so that they would kill the Doppelganger, Yukari would alter the boundary of strong/weak on Nue, and Nue makes the entire group non-identifiable, making it so that the doppelganger can't copy their abilities. Why? Because he now can not comprehend their abilities or their identities, meaning that he can't copy their abilities. They are now unknown people with unknown abilities. The group already knows their opponent, and he probably knows them too, BUT because of Nue's ability, he would not be able to tell it was them because they are now unknown, completely and utterly unknown. She essentially erased their identities.
Yukari alters the boundary of strong/weak on Sakuya and Kaguya, boosting their powers to godly levels. Next, Sakuya and Kaguya accelerates the entire group to unprecedented levels, allowing each person to react so fast that their very reaction time would warp reality. Then, they find the doppelganger and attack him. He would not have the reaction time yet, witch means that the group basically has all the time in the world to stop him. Yukari alters the boundary of strong/weak on Flandre, increasing her power to the galactic scale. Yukari would have enough power because if she was not powerful enough, she could just as easily adjust her power via the same method as Flandre. Flandre then "kyuus" him. No status effects are being used, she just takes his "eye" and crushes it. Simple, but effective. At that exact time, Yukari alters the boundary of immunity/susceptibility, protecting them from any potential negative backlash. As for the counterattack, this is where Seija plays an important role, if Yukari's counter didn't work. When the counterattack is triggered, Seija flips the counterattack, countering the counterattack. The attack would then do the opposite of what it was supposed to do. Instead of erasing everyone, it will do the opposite. They were all alive to begin with, so it would either have no effect, or even make them stronger by reinforcing their existence.
With this, he would not be able to counter because the abilities are unknown. You can't copy something you don't know about.
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