Charge the enemy:
The game sends an infinite amount of troops after the alarm has been set. The best way to defend yourself is to charge ahead as fast, but also as safely, as possible. In some levels you can cut down the number of men by almost fifty. If you are low on ammunition, health, etc., this is a good way to cut down on the amount of men you have to deal with. It also works sometimes if you send a man far ahead and hold him there, then do your objectives for your mission. Once the alarm has been raised, an infinite number of enemies will start originating from certain points in the level such as barracks, campsites, or sometimes level boundary lines. The only way to stop this flow of enemies is to find the point at which they are appearing from and "secure" it. In other words, you have to kill all the enemies in the area (or building) and remain there for a minute or two to make sure no more show up. This way, you can systematically shut down the enemy's "points of origin", then complete the level even after the alarm has been raised.
Hold [Left Shift] and type desertwatch to unlock the cheat menu at the options screen.
Last resort attack on SCUDs:
SCUDs can be blown up with the sniper rifle if they are in the launch position. Shoot the rocket a few times with it.
Last resort attack on tanks:
If you need to take out a tank but have no more long range rockets, it is possible to get close to it without getting hit. First, you will need to have a sniper rifle to zoom in on the top of the tank to take out the gunner, if there is one. After he is down, throw a smoke bomb in the direction of the tank. Try to get the smoke bomb in as close to it as possible. While the smoke is in the air, you can run up to the tank without being hit. The smoke will make it impossible for the tank to see you. Get directly next to the tank and plant a C4 next to it. After it is planted, throw another smoke bomb slightly further away from the tank. Make a mad dash for the smoke. If you are fast enough, you will get behind the wall of smoke and can then make your way to safety. Then, detonate the C4. If you have C4 with a detonator, run to the tank, crouch, plant the C4, then quickly run away. When you get about 10 feet away, use the detonator to explode the C4.
Spreading out munitions:
Do not give too much of the powerful items such as tank mines, C4, anti-tank guns, etc. to a single person. If you have this person advance and he gets taken down by a tank, your other teammates might not have the firepower needed to even get close to the downed person, let alone heal him. It is always a good idea to have each teammate equipped with at the least one high powered explosive in case you need to take something out.
Weak spots on tanks:
All tanks have very heavy front and side armor. However, the tank's rear is lightly armored. Smaller tanks can be hit with a rocket almost anywhere and will usually explode. However, larger tanks must be attacked with an explosive from behind. If you shoot it from the front or side, chances are you will not even make a dent. To take out the tanks with the satellite dish on top, shoot the satellite dish with a sniper rifle and the tank will explode.