When {Oros, the Avenger} attacks you may pay 2W to do 3 damage to each creature without flying. Can you do this more than once of you have the mana or only once per attack?
You may do it once every time it damages the opponent.
RESOLVED
Combat damage, but close enough ;)
Non-white not non-flying
If I may delve a little deeper into this one, the reason you may only use the effect once is because it is a triggered ability, not an activated ability.
With an activated ability, you have a cost, then an effect. In order to do the effect, all that must be done is pay the cost. Cost and effect in activated abilities are always separated by a colon (see: {Centaur Glade}).
With a triggered ability, a certain event must occur, then an effect may happen. While a triggered ability like Oros here may also include a mana cost, you will notice it does NOT have a colon. Because the triggering event (dealing combat damage) only happens once and instantaneously, the ability can only be used once because it needs another trigger to happen again.
Just give it double strike...while attacking with the boros guild leader.