Cumulative upkeep
(At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
At the beginning of your draw step, destroy each creature with mana value equal to the number of age counters on Wave of Terror. They can't be regenerated.
This destroys creatures with mana value exactly equal to the appropriate number, not equal to or less than that number.