Bowblade Assassin

Stockpile {2} (Exile this card from your hand: It gains "{2}: Put a cache counter on this card." and "You may cast this card from exile as long as it has a cache counter on it with its cost reduced by {1} for each cache counter on it.")
When Bowblade Assassin enters the battlefield, you may destroy target creature with power or toughness X or less, where X is equal to the number of cache counters on it as it was cast.