Boreal Elemental

• WWizards of the Coast2019-07-12
  Official Ruling
  An opponent’s spell that targets Boreal Elemental more than once costs only more to cast.
• WWizards of the Coast2019-07-12
  Official Ruling
  To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases (such as that of Boreal Elemental), then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

Rulings via Scryfall