Stopping the Reapers in Mass Effect 3

How is it even possible for Commander Shepard to stop the Reapers in Mass Effect 3? That's one thing I don't like about some of the elements presented in stories today. They always build up this incredible evil/force that is supposed to be so unstoppable and then all it takes is one person to destroy this great evil/force.

So my question is, how is it possible for one human, to take out a force that no other civilization in the history of the Mass Effect galaxy could contend with?

Mind you I have not read any of the books. ;)