Magus of the Arena

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

• WWizards of the Coast2007-02-01
  Official Ruling
  If either target leaves the battlefield before the ability resolves, the ability taps the remaining target, but no damage is dealt. If both targets leave the battlefield, the ability doesn't resolve.
• WWizards of the Coast2007-02-01
  Official Ruling
  If an effect changes the targets of this ability, the second target can be changed to any creature the chosen opponent controls. The choice of opponent can't be changed.
• WWizards of the Coast2007-02-01
  Official Ruling
  Tapped creatures can be targeted. Tapping them again will do nothing, but they'll still deal damage.
• WWizards of the Coast2007-02-01
  Official Ruling
  Magus of the Arena's controller always chooses their creature first.

Rulings via Scryfall