Battle Bracket (Round 1 Pairing 14)

It can be Rouge if you want, but this is just two Pokémon battling

I won't be making it Rouge in my answer

I'm going with Rayquaza

It can Dragon Accent and Mega Evolve right off the bat

Zygarde had to drop to 50% of its health to go into 100 in order to Mega

Plus, as @UnovanZorua said, Rayquaza can learn powerful Ice attacks

Mega immediately then hit with Blizzard and I can't see any chance for Zygarde