If a card in a player's graveyard has in its mana cost, X is considered to be 0.
2014-04-26
Heroic abilities will resolve before the spell that caused them to trigger.
2014-04-26
Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.
2014-04-26
Heroic abilities won't trigger when a copy of a spell is created on the stack or when a spell's targets are changed to include a creature with a heroic ability.
Heroic — Whenever you cast a spell that targets this creature, return target creature card with mana value 2 or less from your graveyard to the battlefield.
in its mana cost, X is considered to be 0.