I'm playing it through on the hardest difficulty, but I can't beat this part. I've tried exhausting all of my sniper rifle ammo from the rock on the the cliff that you start on to no avail, and running around the edge to see if you have to climb up something.
My brother beat it on an easier difficulty and said that you just have to shoot them while in the pit-like area, and eventually you'll be rescued. The pause screen seems to confirm this, but how long does it take?
Do you have to kill a certain number or wait a certain time? How many/long? Also, how far down do you have to go? I've tried hiding behind the first rock and unloading a couple of PPSh-41's, but I still didn't win.