Merfolk Trickster

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

• WWizards of the Coast2018-04-27
  Official Ruling
  If the target creature has power and toughness written as */* with an ability that defines its power and toughness, it’s 0/0 when it loses all abilities. If its power and toughness are written as */*+1, it’s 0/1, and so on.
• WWizards of the Coast2018-04-27
  Official Ruling
  Once you announce that you’re casting a spell or activating an ability, no player may take other actions until the spell or ability has been paid for. Notably, players can’t try to remove a permanent’s activated abilities to stop them from happening. Once activated, the ability on the stack will resolve even if the creature loses the ability.
• WWizards of the Coast2018-04-27
  Official Ruling
  If the target creature has an ability that triggers when it becomes tapped, that ability triggers before it loses all abilities.

Rulings via Scryfall