Post History

All that has been said here about doing research, and about the advantage of the author's omnipotence, is valuable. But there is a more basic answer as well. Don't show how the sausage is made. Your hero is a tactical mastermind. Fine, have men drink a toast to his brilliance in the tavern after the battle. But don't explain the plan and its execution in excruciating detail. That's pretty boring anyway. Fiction is about moral choices. Battles, in faction, as about courage, not tactics. 

And consider how you know that real people have exceptional gifts. How do you know that Einstein or Jonas Salk or Bobby Fisher or Bach or Schrodinger or Socrates or Aquinas were geniuses? Unless you happen to be pretty highly gifted in their respective fields, you know it because you were told it was so and you believed it. How do we know the characters in your story are geniuses? Because other characters tell us so.    

There is a genre is which, to an extent, the actual works of genius is presented, or, at least, it appears that a work of genius is being presented. But there is a reason that people make jokes about Sheerluck Holmes. Much of it is baffle gab. And what remains involves authors who have genuinely made themselves experts in a field, or were already before the decided to become authors. They don't have to fake it because they actually know their stuff. So if you want to be one of them, start studying.