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This is a great example of power creep!

Logically the 0 mana monsters have "infinite" value. In battles were you have space left they simply add chance to win without mana.

Concerning monsterpower; legendary monsters are always stronger (~+1.5)
it has only 1 health, so i think it's on curve...

It's a great reason why the power calculation doesn't tell the whole story! (Atleast for now ;) )