Destroying anti-aircraft guns
1 - starting area
2 - first AA gun
3 - second AA gun
4 - third AA gun
As you've probably noticed, there's a long journey ahead of you. Destroying three AA units is going to be your main objective for the next 15-20 minutes. Make sure to follow my instructions if you don't want to deal with any unforeseen difficulties along the way. Start off by moving south. Destroy or ignore an enemy humvee.
Keep moving south, because that's where you'll find your first AA gun. Eventually you should come across a small barricade. There are a few RPG soldiers out there, so make sure that you've killed them as soon as possible. You should also secure the left hill. Remember that you won't have to stay inside the tank during these fights. It's sometimes better to exit the tank in order to kill enemy soldier with a sniper rifle.
Make sure that the area has been secured before you decide to move on. Get closer to the barricade and go past it. Now you will have to turn right. There's an enemy tank in this area. Obviously you will have to destroy it. Once again, ensure that your tank doesn't receive too much damage.
Leave your tank once you've secured this entire area. I wouldn't recommend using your main cannon to destroy the AA gun. Instead, place a C4 charge on the AA unit. Move back to a safe distance and detonate the charge. Go back to your tank. Our next objective is to the south-east (3 on the map), so you will have to go back in order to get there.