So I am facing Diantha in X/Y, and I have the Pokémon that takes cover of ALL Diantha’s team, and all of them we can mega evolve them, Can we just win with a fully mega-evolved team?
You are limited to 1 mega evolution per fight. You are much better off choosing 1 Pokemon to mega evolve for each e4/champion and give the rest normal held items, otherwise your 5 other Pokemon which you didn't (becuase you couldn't) mega evolve would be holding a useless item.
Source: experience.
