Seashell Dragon

Breeding
You can get this dragon by breeding a Metal Dragon with a Water Dragon at the Breeding Mountain or the Ultra Breeding Tree. Breeding a Metal Dragon with a Water Dragon may also result in a Mercury Dragon.

Armadillo Dragon + Waterfall Dragon

Damage Calculator
To explain, when attacking with Water, Seashell deals double damage against Fire and Electric, medium damage against Water and Ice, and no damage against Plant. When attacking with Metal, Seashell deals double damage against Earth and Ice, half damage against Fire and Metal, and no damage against Electric. When defending, Seashell takes double damage from Fire and Dark, medium damage from Water and Plant and take no damage from Earth.

Counter Dragons
These are dragons that are best suited to defeat this dragon in Combat and there can be any number of good counter dragons:

Trivia

 * In its first stage, the Seashell Dragon shares traits of Clams and shellfishes. It lives inside the shell, being a part of it and can hide within. As it grows, the two hinges of shells become its wings.