Emberheart Challenger

• 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
  Valiant — This ability triggers whenever this creature becomes the target of a spell or ability you control.

• WWizards of the Coast2024-07-26
  Official Ruling
  If a spell or ability you control has one or more of its targets changed to a creature you control with a valiant ability, that ability will trigger if that creature hasn't yet been the target of a spell or ability you control this turn.
• WWizards of the Coast2024-07-26
  Official Ruling
  Valiant abilities will resolve before the spell or ability that caused them to trigger.
• WWizards of the Coast2024-07-26
  Official Ruling
  If you create a copy of a spell on the stack and target a creature you control with a valiant ability, that ability will trigger as long as the creature hasn't yet been the target of a spell or ability you control this turn.
• WWizards of the Coast2024-07-26
  Official Ruling
  You pay all costs and follow all timing rules for cards played this way. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.

Rulings via Scryfall