Try using storm drain Gastrodon. Also you should consider teaching your roserade natural power. Unless you plan on using Gardevoir as a mega I would think of faster psychic type pokemon such as Alakazam, Espeon, or A-Raichu.
Mega Blastoise is a more adaptable choice. Seeing how once you get your first shell smash in you're pretty set against most mons without priority.
Blastoise @ Blastoisinite
Ability: Rain Dish(Mega Launcher)
EVs: 252 Spa / 252 HP/ 4 Spe
Modest Nature
-Water Pulse/Hydro Pump
-Ice Beam
-Dark Pulse/Aura Sphere/Rapid Spin
-Shell Smash