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 means that opponents' attacks will never land a critical hit. In terms of stats, its previously high Defense jumps still more, and its Sp. Attack also increases, making Mega Slowbro a great Pokémon for long, sticky battles that require endurance.