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I’ve written a full guide about that here.
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It’s really important that you don’t just throw in all your strongest dragons and hope for the best – it’s a lot more effective if you take a closer look to the parent (first) elements of each dragon your opponent has and choose a dragon that will be strong against that dragon, that will let you cycle through your dragons and take out his dragons a lot faster. You could also play it risky and set there and ice or water elemental dragon to hope for the first hit but I personally don’t like gambling here at all and use a pure dragon with high hitpoints to not have any bad surprises at the start and play by battle after the first move.
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Only the first element determines strength and weaknesses, so you can ignore all other elements when it comes to weak/strong.
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