Storm Dragon

Breeding
You can get this dragon by breeding a Water Dragon with an Electric Dragon at the Breeding Mountain or Ultra Breeding Tree.

Breeding an Electric Dragon with a Water Dragon may also result in a Lantern Fish Dragon.

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