Brennan
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However, the situation with Hydras is not so straightforward. I prefer a single-unit in the corner. In this sense, Vamp Lords, Ogre Lords, and even Golems are not bad in a pinch.
In addition to the aforementioned tactic, you should use the "blow-through" ability, that is, an attack on a dragon, as a result of which the counterattack hits another dragon.
By the way, a Hydra costs 800, not 600, as mentioned above, and you'll have to pay 7000 rubles for 2 cooldowns.