Fairy
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And you won't be able to defeat spiderlings in spider form either. Because dark elementals are immune to dark magic. And ghosts, liches, spiderlings, and the hero himself use dark magic.
It's the same as if you put only your dark elementals against enemy dark elementals; the enemy elementals will constantly defend, they won't be able to attack your elementals, just as your elementals won't be able to attack the enemy elementals.