I thought that taking models from the back was a rule anyway? It signifies, basically, that the guys in the front rank die and his comrades charge in to fill the gaps when they go down. And in the end it is a lot easier.
This is a flank attack though - makes it a bit trickier. I am not sure how you would do it if it was a prolonged CC. (mine never are!)
If you continuously remove from the back rank, then sooner or later you will reduce the models in base contact with a flank attack.
Conversely you would not want to reduce the frontage of the front rank, until all models in 2nd and other ranks were removed.
I suppose it may not come to that because of CR forcing a break, or the opportunity to lap round.
I suppose it's possible to have a protracted combat with a non-breaking unit like flagellants, attacking them in the flank, and then I am not sure of the order in which the flagellants would remove models.