Highway of Death
After resupplying and taking out more enemies, APCs will spawn along the left side of the road. Use your sniper to take out their tires. You will need to shoot 3 tires on an APC before it is disabled, and you will need to disable 3 of the 4 APCs as they approach. You have plenty of time, and the shots become easier as they approach because you don't have to compensate for wind or distance as much.
Reload the last checkpoint if you miss too many shots.
Possible on any difficulty, but recruit is probably recommended to make it as easy as possible.