Once again you get to play as Stone. This will be the last mission in the game that you unlock. It's unlocked by getting 100% headshots in everything, including all other bonus missions.
As always, you play this round initially in Story Mode. Again, this only means that you don't get the weapons selection screen at the beginning. Your chance of getting survival in Story Mode is slim to none. It's highly suggested that you take this round in Easy difficulty the first time. Your only weapon is Stone's Rifle, as before. Headshots are easiest with this weapon, but there are no headshot kills required (since you have to get 100% headshots just to unlock this level).
All of the shots are at a distance, making them difficult, topped off with the fact that enemies don't always stand still. Also, bullets tend to be less powerful with distance, so take your biggest guns once you've beaten this round once. Not to mention the fact that Stone somehow gets damaged much more than Gabe or Lian. The BFG is an excellent choice, along with the flare gun. Shotguns have little to no effect, save possibly for the Jackhammer, which again still doesn't work well except for the (relatively) closest enemies. Be sure to use crouch as you did when assassinating Mara, as Stone has a lot of sway with his aim.
There is an ammo refill box on the far end of the balcony (the right as you face outwards). Do NOT refill your ammo after doing battle with the enemies in the towers on that end (the screen will say "2 enemies remaining").
Most of the mission is generally easy enough, but the last wave is where all hell breaks loose. Yes, I'm not bullshitting you; the final two fuckers both have rocket launchers. In the next-to-last battle you should snap to the wall closest to the flak jacket box. When those enemies are dead RUN LIKE HELL to the far end of the balcony, past that box. Walk in just close enough to where you can see the enemy that comes in from the right. He will likely take numerous hits to kill. Use a building to block the shots of the rockets: the projectiles only need to come near you in order to kill you.
When the one on the right is dead, then go for the one on the left. You'll have to poke yourself out just far enough to see him, take a shot, then run back to the left to avoid the blast of the rocket which will hit your balcony. Aim for chest shots, and know that he will likely take several hits to kill (and most often you'll miss anyway).
Because of this mission's terrible difficulty spike at the last wave, I highly recommend powerful weapons like the Desert pistols, Calico HE, SMAW, Barrett 50-cal etc.