The New King James translates "devices" as "wiles." We could also translate this word as "contrivances," "techniques," "stratagems," "plans," "procedures," "plottings," or "schemes." Whichever we choose, he has ways that are designed to achieve a particular goal.
A device, stratagem, technique, or contrivance might be thought of as being a tool to carry out a certain function. But in this context (see the notes at II Corinthians 10:3-5) the implication is that the device is primarily mental. He is clever and crafty. He possesses ingenious subtlety, but he also has a modus operandi that presents us with clues about his influence and tends to give him away, rendering much of his cleverness inoperative and making him easier to defeat.
The idea is to catch him as he begins to use his devices to twist us mentally to agree to the line of reasoning that he wants us to follow. If we can catch it as it begins, we will not be entrapped by it. We know that Adam and Eve did not catch it, and they were led astray.
One of a Christian's primary defenses against Satan, of course, has to be a prior awareness of his modus operandi—particularly his desire to turn good into evil. Perhaps nothing could be more devilish than to do such a thing. And in this very context, Paul suggests that Satan can get to a person through a spiritual quality that is good.