It crushes its foe under its heavy body to cause fainting. In a pinch, it will withdraw inside its shell.
– Pokémon X Pokédex
Mega Blastoise has a Mega cannon on its back; it seems that its two cannons became a giant one and two little ones (hmmm reminds me of Zweilous to Hydreigon with its heads…). I like how the two little cannons on its arms have their own shells though.
Its ability is Mega Launcher, which boosts pulse moves like Dark Pulse and Water Pulse. I wonder if it’ll boost moves without “pulse” in its name, but similar in idea, like Hydro Pump and Hydro Cannon.