My Gyarados attack Kingdra with Dragon rage in Blacktorn Gym, but it's normal effective! I think kingdra's only weakness is dragon, but... what happen?
Dragon Rage does 40 damage no matter what the circumstance (except when doing no damage or when something like Endure or Focus Sash/Band stops you from doing the full amount of damage.)
Trachy is right so this is just extra information Dragon rage does only 40 damage so it couldn't be super effective see here: http://pokemondb.net/move/dragon-rage
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