Without a doubt this is one of the more controversial aspects of spiritual life. I would say, though, that for the contemplative solitary having an enemy is a, well, god-send. At a fundamental level enemies keep us real, keep us in reality - rather like a wall keeps you in reality when you drive straight into it.
When I think about "enemy" I tend to think of those (people, things) that make me fearful of suffering (digest this for a moment). If I feel particularly archetypal I may think of my enemies things like disease and death. But Benedict says that we should always have our death in the forefront of our thinking. So either this is a case of "keep your enemies closer" or Benedict wants the monk to get over their fear of death. My guess is that Benedict wants us to think of death not as an enemy, but rather a wise advisor. Wise because death is not swayed by the petty ego, and thus is able to provide us with a perspective - a final perspective as it were.
But how about people? I have met too many Christians who take Jesus' injunction to turn the other cheek, as a way to refuse to accept the existence of personal enemies. "I love everyone" is their motto. I am sorry to say but this is frequently an anemic form of faith, closer to a moldy dark abandoned basement than a virile and ensolared power which brings light to the world.
My own self-analysis (for what is worth) leads me to believe that this is a particularly pernicious form of egotism. No one loves everyone that way. Jesus did not love everyone that way. He is LOVE, and so was quite capable of calling people "vipers", and Peter "satan"...
Benedict suggests that the purpose of the cenobitic life is to prepare someone (heal the petty ego, strengthen the good ego) to become a solitary and go out to do battle with the devil by themselves. The devil is everyone's real and final enemy, but there are other things to hold as enemies: the prophets did battle with the injustices of society, and they frequently called the king to the carpet, by name! (In this vein, have you ever wondered why the Bible frequently calls nations by personal names, like Ham for Egypt or Israel for the Hebrew people?)
Idea: instead of blaming an amorphous conglomerate like BP for the spill, we should pray at the CEO. Yes, "pray at"! : )
I propose to you that if you are not able to concretely identify at least one real enemy (yes a person, even if he or she is a figurehead), then you are not doing your job of solitary very well.
My challenge to us is this: how aware are you of your enemies? How many enemies can you list? Are there any real people in your enemy list? Can you change the list so that you have actual names (and perhaps even faces - Google them)? How are you doing battle with your enemy? What concrete steps are you taking? Daily? Weekly?
As you walk up to your prie-dieu realize that you are marching up to the front lines. As you pick up your breviary or settle into your prayer word, you are firing a shot at all that keeps people starving, afraid, suffering. All those headlines you see on TV, the newspapers and the internet. Be angry at it. Then look at your enemies and pray at them. Pray with all your might. Do not falter, not for one moment - you are redeeming the world one name at a time, one prayer at a time.
If it helps, think of this exercise as the shadow version of "love your neighbor". If you cannot name three or four of your actual neighbors, then I would say you are simply loving (and living) a fantasy.