2) Wait for Reinforcements
Order your team members to enter this new base. You will be able to rest here for a while. The objective is to wait for the reinforcements to show up. You'll probably notice at least one allied gunship (#1). Sadly, you won't be allowed to take command over new soldiers. Instead, head on to a new vehicle. It's called Mule (#2). This vehicle will allow you to rearm yourself. You will also be able to give it simple commands, so you won't run out of ammo even if you enter the quarry. I wouldn't recommend exchanging a sniper rifle for anything else, because you're going to need this weapon.
3) Clear the Camp
It's time to take on the main objective for this second mission of the campaign. You will have to secure three different camps, located throughout the entire quarry. The closest outpost is located to the north of your current position (#1). As a result, that's where you'll need to go (#2). It's going to be a small hill. The rest of the team should be following your lead.
As you've probably suspected, you'll have to be extremely careful here, mostly because you won't have any cover areas or alternate routes in order to surprise enemy soldiers. Check the first screen, because that's where you'll have to make a stop (#1). There are at least two enemy units to your right (#2). They are standing near a truck.
Lean out carefully. You will have to kill two enemy soldiers (#1). Focus all of your efforts on killing them as soon as possible. Otherwise you would have to pursue wounded soldiers. Once they're gone, move to the left. Take out a single opponent. He should be patrolling a nearby square (#2). Thankfully, his death shouldn't result in raising the alarm.
Start moving towards the left truck (alone). I would recommend that you hide behind this vehicle. Lean out carefully for a better view. You should be able to kill at least two enemy soldiers from here (#1). The rest should be standing at the opposite end of the camp (#2). You will have to wait for them to leave their cover areas.
Make sure that you've killed all enemy units. This will allow you to secure this first (out of three) camp. DON'T go back to the allied camp just yet. Instead, head north (#1). As you've probably noticed, you're going to have an excellent view at the entire quarry (#2). I wouldn't recommend sending your sniper here right away, because he would have been probably shot. You'll have to secure this area without his immediate help.
You will have to kill more than 10 enemy soldiers from here (#1), so obviously I won't tell you exactly where to look for them. Just make sure that you're using the zoom feature and the tactical view to find them. You should also take a out a single sniper. He will be standing to your left (#2). He must die as quickly as possible, because he poses a serious threat to your safety.
The second enemy soldier is standing on a lower level of the exact same construction (#1). You will also have to dispose of him quickly. Once these guys are gone, allow your sniper to take your spot (#2). He should be able to deal with the remaining enemy forces.
Once the area has been cleared, try moving (along with your sniper) to the left. You will end up standing on a metal construction (#1). Allow your sniper to stay here, because he should kill a few additional enemy units. Use the nearby staircase. Obviously you will have to go down to the ground level. Be careful, though. One of the rebel soldiers will probably be guarding the staircase (#2), so you must kill him quickly. Save your game and order the rest of your squad to get here.
Now you should consider moving towards the left camp. Check the first screen if you don't know where it is (#1). As a result, you will have to be moving near a left wall of the quarry. Once you're half way through, you will receive a short transmission from the commanding officer (#2). It turns out that there are a few RPG soldiers in the area. You will also have to find a good cover in order to avoid being killed by them.