Somberwald Stag

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

• WWizards of the Coast2016-07-13
  Official Ruling
  If the targeted creature is an illegal target when Somberwald Stag’s ability tries to resolve, the ability doesn’t resolve. If Somberwald Stag is no longer on the battlefield, the targeted creature won’t deal or be dealt damage.
• WWizards of the Coast2016-07-13
  Official Ruling
  You choose the target of the triggered ability as it goes on the stack, but you choose whether or not to fight as that ability resolves.

Rulings via Scryfall