In the last section of Chapter 3 the objective will change to Cross the Mountain Ruins and you will eventually come to a derelict shack of some sort with three floors. After exploring all floors and collecting ammo (hunting rifle ammo on third floor) you will reach the roof which will then collapse and send you back down to the third floor. There are about 7 or 8 taken in this room when you drop. Just kill them all with hunting rifle and reload a few times. It didn't take me long. You will have to had picked up the hunting rifle earlier in the level but there are one or two different locations where you should find it so it shouldn't be a problem.