(as) slippery as a snake

Devious, scheming, and untrustworthy—and difficult or impossible to apprehend or pin down. The notorious criminal has proven to be as slippery as a snake for police, eluding capture yet again. These companies are slippery as snakes when it comes to regulations. They have all sorts of elaborate means of getting around them. Their embezzling CEO has been slippery as a snake, but we've finally got some hard evidence on him now.