Mega Blastoise is 135 base Sp.Atk, 78 base Speed pokémon that can use Shell Smash to boost it's sweeping capacity much, much more. But for setting up Shell Smash, you have to survive a hit, in order to do that you're gonna have to set up against a pokémon that's (before the set-up) faster than you, so they hit you before your defenses weaken. Modest nature, as, after 2x speed, it can even outspeed Deoxys-Speed. Water Pulse is the option for most damage, and scald for the chance of burning physical attacker that may live scald (and also break Focus Sashes).
