Platinum Dragon

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

Breeding an Ice Dragon with a Metal Dragon may also result in a Pearl Dragon.

You may also breed Dandelion Dragon and Medieval Dragon or breed Robot Dragon and Ice Dragon to get Platinum Dragon.

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