Hardened Berserker

• WWizards of the Coast2015-02-25
  Official Ruling
  The ability can apply to alternative costs to cast a spell.
• WWizards of the Coast2015-02-25
  Official Ruling
  If there are any additional costs to cast a spell, apply those before applying Hardened Berserker and any other cost reductions.
• WWizards of the Coast2015-02-25
  Official Ruling
  Hardened Berserker can’t reduce the colored mana requirement of a spell you cast.

Rulings via Scryfall