There are two spots on the "All Ghillied Up" mission where you can face an army of enemies or practice your shooting skills. Wait until you have passed the convoy with the tanks and the troops. You will see the two guards throwing dead bodies in the water. Shoot one of them in the leg and he will alert others. The entire convoy will then come at you. The other location is at the fire escape. Go up to the floor of the fire escape without shooting the sniper, then get a G36C from the top room and a Stinger. Shoot down the helicopter to get an enemy army after you.