3) Destroy Enemy Artillery
You will only have to destroy two artillery units, however you must explore three caves in order to find the tanks. As you've probably noticed, you will be given control over a flying machine, called UAV. It would be wise to use it to scan the entire area. Head on to a small passageway. It's located to the left of the secured base (#1). Keep moving north. Order your UAV to scan this entire area (#2). You will have to scan two lower caves, because we'll explore them very soon.
You should be able to spot one of the enemy soldiers (#1). He will be guarding the middle cave, however there's nothing wrong about taking him out right away. Head on to the lower cave. It should be located to your left (#2). I'd strongly recommend leaving the rest of the squad behind. Make a second scan with the UAV.
Sadly, you won't be able to see what's inside the cave, because it's very dark here and using a night vision goggles wouldn't help you. As a result, I'd recommend throwing a grenade inside the cave (#1). Once the explosion has occurred, run to the entrance and deal with the remaining enemy units (#2). You won't find an artillery piece here.
Go back to the main passageway and start moving north, towards the second cave. Once you're closer to it, turn left. Once again, all enemy soldiers should be hiding inside the cave and you won't encounter any snipers on your way to the cave (#1). Once you're there, you will receive an information about the visible artillery piece. You should be able to notice it without any problems (#2).
I would recommend that you repeat the attack from the previous cave, so you should try throwing a grenade. All enemy soldiers should be standing to the left of the cannon. Wait for the explosion to occur and kill them (#1). I would recommend leaving the rest of the team behind. Make sure that the area has been secured (#2).
Obviously you will have to get closer to the artillery piece. Press and hold the X key. This will allow to place a C4 charge on the tank (#1). It's going to take a few seconds, so you must be patient. Once this is done, move back to a safe distance. Press the X key once again in order to detonate the charge (#2). Go back to the main passageway and start moving towards the final (third) cave. The rest of the squad should be following your lead.
You could also order your UAV to scan this area. This will allow you to locate one of the rebel units and you'll be able to take him out without even seeing him (#1). You could also throw another grenade, however this won't be necessary. I would recommend that you allow the rest of the team to take part in these fights. You will have to secure this cave (#2). Thankfully, it's not going to be well defended, so it shouldn't be a huge problem.
Now you will have to order your men to leave the cave, because you will have to destroy the tank. Head on to the artillery piece. Place a C4 charge on the tank (#1). Just as before, you will have to move back to a safe distance. Blow up the tank in order to finish up with this mission objective (#2). Wait for some new orders to be displayed on your screen.