Hydra is still one of the strongest tanks despite his health reduced from 12 to 10 at max level. This cards abilities are the one all tanks need. It's important for a tank to have healing ability because tanks are most of the time under attack and they need to resist to protect all friendly monsters behind him. Also with thorn ability he will be able to exacute enemies without attacke. Retailate ability most of the time is better than double strike because monsters with retaliate ability have strong melee attack. Trample ability is also helpful. Despite it's the most expensive card it's worth to buy this card to see how strong monster he is.
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From @smalien