Crustacean King is a must have card in water team because he is the only protector of water team and defense is important in strategy games. He gives 2 shield to all friendly monsters where at least all friendly momsters will be saved from enemy's first attack. He don't just protect monsters also he heals tanks which is a great ability. Reasons to not play with this card are it's a most expensive common card, his attack is too weak and additionally slow speed makes his attack more weaker.
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From @smalien