I found out the problem as to why no one can complete Mountian Pass.
When you first accept the mission, you run up the hill to go into the mission. Once you've entered the mission and have "Beamed" in, all you need to do is run up that first hill, run past the first group running down the hill, and enter the first Camp, you'll notice that the Mission Dialog will up date with what you need to do, once this is done, go back to the first group and kill them. Then play the rest of the mission like you've done before.
I've done this three (3) times now and it's worked everytime.
DEVS... You need to move the Activation point for the Mission progression activation spot closer to the enitial spawn-in point apon entering the mission. Before the First Group can be engaged.