His greatest victory is in those he's deceived into believing that he doesn't even exist.
Or we see "him' for what he is, a literary construct.
Here's the problem I've always had with Satan as a concept, reading the bible. Is he God's enemy or one of God's enforcers?
The bible seems to indicate he's both. One one side, he's tempting people into sin, on the other, he's doing God's dirty work of punishing the guilty.
Now, keeping in mind that most of our western view of Satan is molded by Dante and Milton, rather than the Bible, let's look at what the Bible says.
In the book of Job, Satan comes and goes as he pleases, and has God's permission to punish Job horribly so that God can prove a point. What point was that? Not really sure. Job demands and explanation, and God's answer was, "I'm God, I can do whatever I want. Who are you to question me." Job sure would have been pissed to find out Satan killed his whole family on a bar bet with God.
Or we can look at the First Book of Chronicles, that said Satan tricked David into taking a Census (as opposed to the 2 Book of Samuel, which said that God ordered the census).... but regardless of who made David take the Census, it was God who went all Batman Villain and made David choose between War, Famine or Pestilence, and when David picked Pestilence, thinking this would be the least bad of the three, 70,000 Israelite died.
I'm sorry, which one is the bad guy in this story again? I forget.