I believe I read this in the mid-to-late 1980s, quite possibly in Analog.
I don't recall the justification, but legal disputes (as between corporations or countries) are sometimes resolved in a form of virtual trial by combat, where 2 professional gamer-players operate either side of a historical conflict. The battles might be well-known ones, like Cannae, or lesser-known battles, and be from any era up 'til the Renaissance.
The protagonist is a gamer who has some kind of possibly-psychic, possibly-imagined link to a deceased friend of his who gives him background knowledge and advice. It's not clear that this gives him a real advantage over others, though; he does well but is not dominant. The friend might be named "Richard;" either that or in one of the battles he fights as King Richard.
I recall there being some games shown before the final one that forms the climax of the story. One of them might be Cannae, or Carrhae, or another one of the classic battles where cavalry was dominant; cavalry tactics may have been one of the weaknesses of the protagonist.
The climactic battle is a tough one; the protagonist is given the side that historically lost. I believe he still lost, but in a new and interesting way that suggested he could almost have won if he'd thought of whatever his tactic was a bit faster.
This is quite different from Interstellar culture resolves disputes with simulated duels; antagonist cheats using telepathy. The story takes place on a near-future Earth and there is no suggestion of cheating.