Game Changer

Hexproof
Game Changer can never untap.
In the casting of all your spells, including this,
destroy any number of any opponent's creatures.
The spell you casted costs X less mana of any
color, where X is half the number of creatures
destroyed in this way(rounded up).
Each time a creature is destroyed, draw a card.
You may cast it right away for no cost.
{t}: Gain 50 life. This can be used once a game.