Later in the game you will encounter a courtyard. Take out all the snipers and kill the remaining enemies. After you kill all of them, a red truck with a 50 cal on it will approach from the road ahead. After taking out the .50 cal, go across the street to the library and into the computer store. Looking through the window, throw a smoke grenade at the tank. The tank now cannot see you. Get in position and take out your M203 or grenades and start firing. It takes about four grenades -- do not wait too long or the smoke will wear off, and the enemy will open fire.
After you venture down the hole in the building, you should get to an old subway after you explore the sewers. Once you kill all of the men down there, do not go through the door. Keep going strait through and up a catwalk. Before people in the subway notice you there, tell your men to hold their fire. At the bottom of the catwalk on the other side, you will see a yellow generator. Shoot it, and it should kill the lights. Put on your night vision goggles but be careful; they will still shoot at you but will usually miss. Tell your men to open fire and move through the subway to your next checkpoint.
The key to this game is aggression and your brain. If you are aggressive and poke your head out and see an OPFOR, stay there and shoot unless there are more than three OPFORs. If you look and pull back, they know you are there and have time to scatter and move to positions; do not let this happen. If you poke your head through and find enemies, stay there and fight or they will move to positions that will make it difficult for you and your team to flush them out. Also, suppressive fire can be very useful. Have a Devildog lay some fire down while you and your team get in position. Listen to what your teammates say. Slow is smooth, smooth is fast. Before you enter a room, take the time to check and see if you need to reload, if your teammates need health, or if they are reloading. Do not enter a room unprepared. Keep in mind that there is opposition behind every door, down every street, and on every rooftop and balcony, no matter where you are. It also does not hurt to throw a grenade in a room that you do not think has anybody inside. Finally, do not rush through the game, as it is not a race to see who can finish the fastest.