What did Paul mean when he said he was not ignorant of Satan's devices?

Paul meant he was not unaware of Satan’s devices because his own past experience had taught him how Satan operates. He had evidently endured times when Satan seemed to beat him down into despair, so he spoke from firsthand knowledge rather than ignorance about the enemy’s methods.

Les explains that Paul’s statement means he is not ignorant of Satan’s devices; he will bring in his own past experience, when Satan had seemingly almost beaten him down in despair. Paul’s awareness of Satan’s tactics was therefore not merely theoretical but personal. In the surrounding context, Les also stresses searching the Scriptures and running from anything not taught in Scripture like a plague.

2 Corinthians 2:11

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

What did Satan almost do to Paul in his past experience?

In Paul's past experience, Satan had almost beaten him down in despair. Paul recalled this when he said he was not ignorant of Satan's devices. The context presents this as a personal example of how Satan nearly overwhelmed him in his ministry.