Burst of Strength

When you cast Burst of Strength, put X time counters on Burst of Strength.
At the beggining of your upkeep, remove a time counter from Burst of Strength.
When you remove the last time counter, sacrifice Burst of Strength.
Enchanted creature gains +3/+3
Enchanted creature attacks each turn if able
{4}: Pay {e}{e}{e}{e}{e}: Put one time counter on Burst of Strength. This ability cannot be activated more than once a turn.