Plutarch's thesis is essentially the same as the "Great Man" theory of history. To argue against this point of view, we should argue that A) historical circumstances bring out the greatness in people and B) great forces in history are more important than individuals.
On the first point, we should reflect on the individuals who we see as great leaders. These are people like Washington, Lincoln, and Franklin D. Roosevelt. All of these people had greatness forced on them, in a sense. They were the people who happened to be leaders in times of crisis. They may have changed history, but only because historical circumstances gave them a chance to do so.
On the second point, we can argue that individuals do not determine history as much as larger forces do. For example, we can argue that World War II would have happened with or without Hitler. Japan and Germany would have, regardless of who their leaders were, felt angry about the world order in the time after WWI. They would have wanted to expand their power. The other countries of the world would still have responded as they did. This means that Hitler was not the one who drove history.
Of course, none of this can be proved. We will never be able to know what would have happened without Hitler or if some other president would have handled the Civil War better than Lincoln did.