In the Amillennialist understanding the thousand years are symbolic and refer to the period of time between the Lord's Ascension and His future Parousia. It refers to the fact that Christ is, right now, King of kings and Lord of lords, enthroned at the right hand of the Father, having been given eternal kingdom and dominion. And the primary way we experience Christ's kingdom is in and through His Church: as the Gospel is proclaimed, sins are forgiven, and the lowly are loved and cared for.
The devil is seen as defeated already, when Christ suffered and died once for the sins of the world He has defeated sin, death, hell, and the devil. The concept of the Harrowing of Hell and Christ's descent into hell means victory is already attained, by the Lord Jesus, having overthrown the devil's kingdom. So when we read in the Apocalypse that the devil was bound in the bottomless pit for a thousand years, and also that we read that he rages because his time is short, those are not seen as sequential, but simultaneous. The devil is defeated, and he rages. St. Paul says he is like a roaring lion seeking to devour whomever he can; at the same time St. James writes that if we resist the devil he flees. The image of the roaring lion is an interesting one, it is the image of a lone scavenger. Male lions, without a pride, are lone, wandering the wilderness, and will eat whatever they come upon. The devil is not depicted as a proud ruler of a pride of lions, but as a defeated lone lion wandering on the outskirts, who is starving and will eat anything because he is desperate and weak.
Remember the Disney movie The Lion King? The character of Scar? That's the devil, out among the bones, a scavenger, emaciated, loud and a coward. A liar, a schemer, and utterly pathetic.
Christ has conquered, the devil has been overthrown, death is defeated, Christ is risen. The devil's time is short, he is loud, he is angry, he rages--but we can endure his rage because we have Christ, and in Christ we are more than conquerors.
-CryptoLutheran