Hunt the Weak

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

• WWizards of the Coast2018-01-19
  Official Ruling
  You can't cast Hunt the Weak unless you choose both a creature you control and a creature you don't control as targets.
• WWizards of the Coast2018-01-19
  Official Ruling
  If either target is an illegal target as Hunt the Weak resolves, neither creature will deal or be dealt damage.
• WWizards of the Coast2018-01-19
  Official Ruling
  If the creature you control is an illegal target as Hunt the Weak tries to resolve, you won't put a +1/+1 counter on it. If that creature is a legal target but the other creature isn't, you'll still put the counter on the creature you control.

Rulings via Scryfall