Why did Paul say he was not ignorant of Satan's devices?

Paul was not ignorant of Satan's devices because he spoke from his own past experience, when Satan had seemingly almost beaten him down in despair. He therefore knew the adversary's tactics firsthand, rather than treating spiritual opposition as something unfamiliar or theoretical.

Les explains that Paul's statement flows out of his personal history. Paul says he is not ignorant of Satan's devices because he is going to bring in his own past experience, when it seemed Satan had almost beaten him down in despair. In the surrounding narrative, Paul is linked with travel from Choas into Macedonia and had arranged to meet Titus, one of his fellow workers in Asia Minor, at Choas. This context shows Paul speaking as one who had encountered Satan's opposition firsthand.

2 Corinthians 2:11

Lest Satan should get an advantage of us: for we are not ignorant of his devices.

What did Paul mean when he said 'we are not ignorant of his devices'?

Paul meant that believers are aware of Satan's cunning tactics and must not let him gain an advantage. The context ties this to restoring a fallen brother: failure to restore gives Satan an opportunity. Satan can masquerade as an angel of light, appear to promote Scripture, and lead many astray.