We’re forgetting that he is not alone. He has a lot of fallen angels helping him. There’s a lot more truth than we realize in the cartoon picture of a good angel sitting on one shoulder and a bad angel sitting on the other.
The devil is neither omniscient nor omnipotent. He is a created being, and only the Creator is omniscient and omnipotent. He is the head of a large organization of evil, and he has plenty of minions who collect intelligence and carry out orders.
If I may be allowed an analogy out of my own life, I had a very, very small role in the first Gulf War. Saddam Hussein, as the leader of the “forces of evil” on the other side, did not even know I existed. However, if he had had an agent in the area where I was living, he would have known what I was doing and could have taken me out, if he had been ordered to do so.
In the same way, we are, for the most part, very low-ranking soldiers in the Kingdom of God. Satan, as the CINC of the forces of evil, certainly knows who Pope Benedict, or Father Corapi, or (on the Protestant side) Billy Graham are, but he most likely doesn’t know me from Adam’s off ox. However, some of his minions certainly know who I am, and what my weaknesses are. This required no omniscience on the devil’s part.