I think the conclusion of the Battle of Castle Black, where things look pretty bad for the Watch/Jon Snow until Stannis appears and saves the day, will be the penultimate episode. I think they will treat it like the Blackwater episode where they focus only on the Wall, but then they will shock viewers in the last couple of minutes by showing Tyrion getting out and shooting Tywin. I picture those last two or three minutes to be completely silent, minus the sounds of Tyrion climbing through the secret passages, until a final dialogue between Tyrion and Tywin. Tyrion will shoot him, watch him die, and then the episode will end. It will be the icing on the cake for viewers that had no idea what was coming or how the Night's Watch/Jon Snow would survive.
The final episode would tie things up with the other storylines, like Jon Snow being elected Lord Commander and being offered Lordship of Winterfell, etc. But then the HUGE twist would shock viewers by showing Lady Stoneheart as the last scene of the season.
At least that's how I would play it.