Slitherwisp

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

• WWizards of the Coast2020-04-17
  Official Ruling
  If an effect allows you to cast a spell “as though” it has flash, Slitherwisp’s last ability won’t trigger.
• WWizards of the Coast2020-04-17
  Official Ruling
  In a Two-Headed Giant game, Slitherwisp’s last ability causes the opposing team to lose 2 life and you to draw one card.
• WWizards of the Coast2020-04-17
  Official Ruling
  An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
• WWizards of the Coast2020-04-17
  Official Ruling
  Players can cast spells and activate abilities after the triggered ability resolves but before the spell that caused it to trigger does.

Rulings via Scryfall