Here is a guide to clear this mission with only 2 players. I did all co-op missions with my friend, but this one was the hardest for us.
I used a M107 sniper and my friend used his favorite rifle (use whichever you normally use). At the start of the level, I went straight and left, on the first opening that leads to the middle of the map. Here, I stood still and waited for the helicopter to take off. Once you do it a couple of times, you will know when the helicopter is triggered, and where you must aim to take it down.
Now, with only one M107 clip, you can take down the chopper. Just shoot 5-6 bullets and watch it burn ;) Now aim left and you will see the second ADATS that the convoy will encounter. Shoot at it from afar and you will eventually take it down. However, some enemies will start to come to your location, including snipers, so be aware of your surroundings and try to stay alive by backing up in the level if necessary.
While you do this, your friend has to clear the way to the first ADATS by going straight on the path. He should use rocks as cover, especially at the end where a turret mounted jeep will spawn. He must take down all enemies and the jeep.
At this time, you will probably be able to regroup near the first ADATS. Take him out with grenades or the M107 sniper.
OK, that was the hardest part. Now you can advance, clearing the surroundings of the factory. One of you (the sniper) should be on the look-out, for there are 3 snipers on the map, and they will headshot you. Once they are dead, you can breathe. One of you has to blow up the factory by going inside and planting C4.
Now regroup near the blowed-up second ADATS and make your way to the third one. Again, you will be able to kill it from afar with the sniper (there is an ammo crate near the 2nd ADATS for you to fill up your ammo).
Now slowly make your way to the end of the level, but make sure no guard is following the convoy. Kill every guard on your way and stand near the rocks at the end of the trail. These will have to be blown up, after which the convoy will escape and the mission will end. Well done ! You just completed the hardest 2-player co-op map for GRAW2.