The full text is...
この魔剣があれば… メルヴィンの言うとおり 竜すらも恐るるに足らない…
Is this a double negative? "Even dragons to be not afraid is not enough" = "Even dragons are [will be] afraid".
I see only one negation in your sentence, and therefore I do not know what made you think that it might be a double negative. Anyway, 恐るるに足らない means “not worth fearing.”
Strictly speaking, 恐るるに足らない is not very correct because it is a mixture of the classical and modern Japanese grammars. As I explained in an answer to the question which cypher linked to, it should be 恐るるに足らず in the classical grammar and 恐れるに足らない in the modern grammar.