Assassin's Strike

• WWizards of the Coast2012-10-01
  Official Ruling
  You must be able to target a creature to cast Assassin’s Strike.
• WWizards of the Coast2012-10-01
  Official Ruling
  If the creature is an illegal target when Assassin’s Strike tries to resolve, it won’t resolve and none of its effects will happen. Its controller won’t discard a card.
• WWizards of the Coast2013-07-01
  Official Ruling
  If Assassin’s Strike resolves but the creature isn’t destroyed (perhaps because it regenerated or has indestructible), its controller will discard a card.

Rulings via Scryfall