Arena

• WOTC Official Reminder Textofficial rules
  Fight — Each of those creatures deals damage equal to its power to the other.

• WWizards of the Coast2004-10-04
  Official Ruling
  Either or both creatures can be tapped prior to using the ability and it still works.
• WWizards of the Coast2004-10-04
  Official Ruling
  The power of the creatures when the effect resolves is used to determine how much damage is done.
• WWizards of the Coast2004-10-04
  Official Ruling
  In multiplayer games, you can choose a different opposing player each time it the ability is activated.
• WWizards of the Coast2006-09-25
  Official Ruling
  If either target is illegal at the time the ability resolves (whether because it has left the battlefield, because it has gained Shroud or Protection, or for any other reason), the ability taps the remaining target, but no damage is dealt. If both targets are illegal, the ability doesn't resolve.

Rulings via Scryfall