決して_ない (or other negative conjugations) could be thought of like "absolutely not" or more literally "decidedly not". It's always used with a negative.
You could interpret this sentence as "An opponent Humans are absolutely incapable of winning against." It wouldn't be wrong to say "An opponent Humans are never able to win against." So in that case, 決して could be translated as 'Never' but the negative part is the ぬ at the end making the whole grammar phrase negative. it isn't a double negative, both ends of the phrase 決して_ない are working together to make one negative.