Dromoka's Command

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

• WWizards of the Coast2015-02-25
  Official Ruling
  As the spell resolves, follow the instructions of the modes you chose in the order they are printed on the card. For example, if you chose the second and fourth modes of Ojutai's Command, you would gain 4 life and then draw a card. (The order won't matter in most cases.)
• WWizards of the Coast2015-02-25
  Official Ruling
  You choose the two modes as you cast the spell. You must choose two different modes. Once modes are chosen, they can't be changed.
• WWizards of the Coast2015-02-25
  Official Ruling
  If all targets for the chosen modes become illegal before the Command resolves, the spell won't resolve and none of its effects will happen. If at least one target is still legal, the spell will resolve but will have no effect on any illegal targets.
• WWizards of the Coast2015-02-25
  Official Ruling
  Although most instants and sorceries that cause damage to be dealt are the sources of that damage, a few cause other objects to deal damage (for example, spells that cause creatures to fight). The first ability of Dromoka's Command won't prevent damage caused by those other sources.
• WWizards of the Coast2015-02-25
  Official Ruling
  You can choose a mode only if you can choose legal targets for that mode. Ignore the targeting requirements for modes that aren't chosen. For example, you can cast Ojutai's Command without targeting a creature spell provided you don't choose the third mode.
• WWizards of the Coast2015-02-25
  Official Ruling
  You can target any player with the second ability, including one that doesn't control an enchantment.
• WWizards of the Coast2015-02-25
  Official Ruling
  If a Command is copied, the effect that creates the copy will usually allow you to choose new targets for the copy, but you can't choose new modes.

Rulings via Scryfall