Mega Mewtwo in the Genesect movie isn't even the same as the one that fought and could barely handle Zygarde who in complete form is one shotted by Ultra Necrozma
Them being different didn't matter during scaling, we use species profiles, not separate, unless of course you want to separate all pokemon media and have like 3-4 profiles for every pokemon.
Mega Mewtwo was severely weakened by being mentally fused with Blaine and had no access to his stronger Y form. Despite that MM took multiple direct point blank hits from Crunch and two Dragon Pulses to the faces and didn't get knocked out (he didn't de-mega evolve which always happens when you're defeated).
Also when the heck did Zygarde get one-shotted by UN? Even 50% managed to get hits off on UN and actually damage it.
Either way he'd scale above base Necrozma who did the 3-C feat.
Mega Mewtwo in the Genesect movie isn't even the same as the one that fought and could barely handle Zygarde who in complete form is one shotted by Ultra Necrozma
Galacta Knight is stronger than Void Termina, Kirby can beat Galacta Knight and Morpho Knight, and Fecto Soul is stronger than Kirby, then Kirby gets stronger than ever and Fecto Elfilis can damage Kirby and send him flying as there are animations of that
None of that proves one-shots.
Galacta knight is not stronger than Void Termina, he literally had 4 knights near equal to him fighting at his side, did you not notice that there's quite literally 4 spears in the Jamba heart? It's outright explained that it was the work of 4 heroes.
Morpho knight is ≈> Galacta Knight and below Aeon Hero Galacta Knight
Kirby is above Morpho knight and ≈ Void termina
Fecto Elfilis can damage Kirby and thus is <≈Kirby
So
VT ≈ Kirby => Fecto Elfilis
Kirby < Massive 3-C explosion
Fecto downscales from that feat