1) Take over the Entrance
Like always, this mission is going to be begin with a short cut-scene (#1). As you've probably noticed by now, there's only one road leading to your destination (quarry). Order your men to follow your lead and start moving forward. Activate the tactical view. You should notice that some of the rebel soldiers are heading towards a nearby vehicle (#2).
Head on to your left. Use the sniper rifle and zoom in on the vehicle. You will have to wait for the rebel units to get there (#1). Kill the first soldier. Once he's gone, take care of his colleague (#2), however some of your team members may kill him as well. Either way, these soldiers have to die. Order your squad members to stay here.
I would recommend that you head on to the northern part of this passageway. Check the first screen if you want to find out more (#1). You should be quite safe there, mostly because these rebels aren't carrying any sniper rifles. You should be able to notice two enemy soldiers (#2). Take care of this small problem. Gather the rest of the team around your position and head south.
Keep heading south-east. You shouldn't encounter any new units for the time being, so you may speed up a little (#1). Once you've reached a junction, you will have to turn left, because you wouldn't be able to enter a large tunnel. I'd recommend slowing down. Start off by sending your squad members towards an excavator (#2). This is an ideal cover point, so they should stay there. Make sure that your sniper is guarding this road, so he should more exposed.
Before you'll be able to proceed any further, you will have to take care of an enemy sniper. Check the first screen (#1) if you want to find out exactly where he is. Sadly, you will have to move to your right in order to be able to kill him. Also, take out a single enemy unit. He should be standing in front of you. Use your rifle to dispose of an enemy sniper (#2).
I would recommend that you head on to your right. You should be able to find a good cover here (#1). There's at least two-three rebel soldiers ahead. Kill them right away (#2). Most of these thugs will appear near a large gate. Remember that you won't have to be in a lot of hurry. Choose your targets carefully and use the tactical view as often as possible.
Most of the enemy soldiers will be hiding near the entrance to this small camp. Check the tactical view to find them (#1), so you'll know where to shoot. Try getting closer to the gate. You must be VERY CAREFUL here. Kill all nearby troops (#2). You shouldn't have any problems doing that. Just make sure that you aren't attacked and wounded by any of the rebel soldiers.
Try moving even closer. Use the metal gate for much needed cover (#1). This will allow you to eliminate all of the remaining enemy forces (#2). Make sure that you have used the map to find them. Wait for some new objectives to be displayed on your screen.