Board Thread:Tips and Tricks/@comment-10773451-20130922190339/@comment-24226549-20131128223326

First of all, you should stop using your Metal Dragon to combat a Hedgehog Dragon as Dark types are super effective against Metal (AKA 2x normal damage). To defeat him with a Metal Dragon, you must try to land the first hit on him by using some sort of Metal attack (e.g. Rust). To do that, you can sacrifice one of your other dragons by letting it die and switching out to your Metal one. This lets you land the first hit on him, but you must be able to kill him in one shot if he has the ability to kill you in one shot. If your dragon isn't strong enough, try training another dragon that doesn't take so much damage from him or use a War Dragon for double damage against him.

These elements take half (or less) damage from Terra: Terra, Metal and Sea.

These elements take half (or less) damage from Dark: Ice and Dark.

I hoped this helped you out. If this isn't enough, you can try to look around the Wiki for more info on those elements.