This expression refers to people, not spirits who do not have human bodies. (See: rc://en/ta/man/translate/figs-synecdoche)
“Darkness” here is a metonym for Satan’s rule or kingdom. AT: “the areas of the universe where Satan is in control” (See: rc://en/ta/man/translate/figs-metonomy).
Christians should use the protective resources God gives them in fighting the devil in the same way a soldier puts on armor to protect himself against his enemies. (See: rc://en/ta/man/translate/figs-metaphor)
The words “stand firm” represent successfully resisting or fighting something. AT: “so that you may be able to resist evil” (See: rc://en/ta/man/translate/figs-metaphor)