>>58030861
I'm not really worried about it being too strong. If nobody else is, then the easiest route would be:
>Base the Mega off of Zygarde-Complete with +100 to its stats for a grand total of 808.
>Allow Zygarde 10% and 50% to both the mega stone to reach this form, with no ability requirement.
>Give Mega Zygarde Aura Break or some other ability (not Power Construct).
Boom, done.
It basically nearly every mega rule (gains way more than 100 base stats, HP skyrockets) but its simple and straightforward.