Wildfire Elemental

• WWizards of the Coast2019-07-12
  Official Ruling
  The last ability of Wildfire Elemental triggers once for each event in which an opponent is dealt noncombat damage, regardless of how much damage that player is dealt.
• WWizards of the Coast2019-07-12
  Official Ruling
  Wildfire Elemental's triggered ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't get +1/+0.
• WWizards of the Coast2019-07-12
  Official Ruling
  Combat damage is the damage that's dealt automatically by attacking and blocking creatures. Any other damage is noncombat damage, even if it's dealt during a combat phase by an attacking or blocking creature.
• WWizards of the Coast2019-07-12
  Official Ruling
  In a multiplayer game, if a source deals damage to multiple opponents at the same time, the last ability of Wildfire Elemental will trigger that many times.

Rulings via Scryfall