Oh boy, that was a 4.0 feature? It was what survival trial used to be before it was updated to the single map. You did 12 battles (you could skip a majority of them) and then you did breakthrough. That was another 8 battles or so and you got more battles with the higher your level was. So at level 50 you were only able to do 1 battle, 55, 2 battles, level 60: 3 battles, and so on.