Pearl Lake Ancient

• WOTC Official Reminder Textofficial rules
  Prowess — Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.
• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2014-09-20
  Official Ruling
  Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
• WWizards of the Coast2014-09-20
  Official Ruling
  Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.
• WWizards of the Coast2014-09-20
  Official Ruling
  A spell or ability that counters spells can still target Pearl Lake Ancient. When one resolves, Pearl Lake Ancient won’t be countered, but any additional effects of that spell or ability will still happen.
• WWizards of the Coast2014-09-20
  Official Ruling
  Any spell you cast that doesn’t have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won’t cause prowess to trigger. Playing a land also won’t cause prowess to trigger.

Rulings via Scryfall