If an enemy exaggerates your faults and a friend underestimates your virtues, then that will help you get a realistic view of yourself. And maybe of your enemies and friends too. The quote is from "The Godfather." Are you reading that book?

This quote is saying that it is very helpful to have an enemy who thinks that you are weaker than what you really are. This way, you can surprise them with strengths that they did not know about.

There would not be better benefit (advantage) in life than having an enemy (who does not love you) who see your faults (mistakes) much bigger than they are, unless you don't have a friend who will appreciate your virtues. So, in life is important to have good friends. They will be always next to you. And than you should not pay attention on enemies :)