Type: Water/Psychic
Classification: Hermit Crab Pokémon
Height: 2.0m
Weight: 120.0kg.
Ability: Shell Armor
Mega Slowbro's energy has gathered in the Shellder clamped onto its tail. As a result, the hardness of its shell is boosted, making an armor so hard that nothing and no one can scratch it. When Mega Slowbro battles, it stands up on its tail. It can move by bouncing along, using its tail as a spring.
After Mega Evolving, its Shell Armor Ability protects it from critical hits. It's already high Defense stat is still boosted even further and its Sp. Attack also increases. This makes Mega Slowbro a great choice for long, dragging battles that may require survivability and endurance. You can even make Mega Slowbro even more resilient by using its Amnesia move, which greatly increases its Special Defense.
Reference and images: Serebii.net
Additional references, images, and information: pokemonrubysapphire.com