Cross-century talents are those whose contribution to society is so great that their accomplishments and lives are remembered and revered through the ages. Cross-century talents should be intelligent, wise, humane, and just. They should also contribute lasting and valuable additions to the world. By this definition, Confucius, Aristotle, and Einstein would qualify as cross-century talents. However, Genghis Khan and Hitler would not, because they were neither humane nor just. David Hume and Bertrand Russell also would not qualify, as they (arguably) did not provide concrete, lasting contributions to society.
Cross-century talents are needed for the progress of humanity. They serve as examples the potential of human beings. They also provide governmental, scientific, and technological breakthroughs that improve the quality of people's lives.
While it is good to strive to be a cross-century talent, such an honor is reserved for a select few. Those that are considered cross-century talents probably do not think of themselves as such, and certainly do not spend time instructing others how to emulate them.
One should simply do one's best with what one has, and try to be a good person. That is probably what the cross-century talents do, too.