You had to vote on the Battle Arena fights back when they were ongoing, chances are if you didn't vote they'll either release it as DLC or you've lucked out and you'll have to trade a code from somebody else
the last I've heard was that the final round was delayed. I'm guessing it will be at least a week before anybody gets theirs although there could be some people getting screwed out of theirs if they had a problem with the site.
I hope I'm wrong. there's also the chance this gets released as free DLC anyway later on. I don't really care about Arrow that much but knowing I'm supposed to have it is making me OCD about it.