Illusory Ambusher

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2017-11-17
  Official Ruling
  Creatures may be dealt damage greater than their toughness. For example, if a source deals 5 damage to Illusory Ambusher, you'll draw five cards.
• WWizards of the Coast2017-11-17
  Official Ruling
  If Illusory Ambusher blocks a creature with trample, that creature's controller can assign any amount of damage from 1 to that creature's power to Illusory Ambusher. Excess damage doesn't have to be assigned to the defending player.

Rulings via Scryfall