Soul-Scar Mage

• WOTC Official Reminder Textofficial rules
  Prowess — Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.

• WWizards of the Coast2017-04-18
  Official Ruling
  If multiple prevention and/or replacement effects are trying to apply to the same damage, the controller of the creature that would be dealt damage chooses the order in which to apply them. As examples, an opponent could choose to apply the effect of Djeru’s Resolve and prevent the damage rather than put -1/-1 counters on it; or an opponent could choose to convert the damage to -1/-1 counters before Insult could double it, and then Insult’s effect won’t apply anymore.
• WWizards of the Coast2017-04-18
  Official Ruling
  Soul-Scar Mage’s effect isn’t a prevention effect. Damage that can’t be prevented will be replaced with -1/-1 counters.
• WWizards of the Coast2017-04-18
  Official Ruling
  If an effect causes a creature you control to fight a creature an opponent controls, the damage is dealt simultaneously. For instance, if your 2/2 creature fights an opponent’s 4/4 creature, your creature will be dealt 4 damage and your opponent’s creature will have two -1/-1 counters put on it.
• WWizards of the Coast2017-04-18
  Official Ruling
  If a spell or ability an opponent controls causes a source you control to deal noncombat damage to a creature an opponent controls, that damage will be replaced with -1/-1 counters.

Rulings via Scryfall