-Always try a stealthy approach rather then using guns. If you are using the Green Beret, you can sneak up on the enemy and cut him. Remember to pick up his items. This will not alert other soldiers unless the "sound bubble" appears, which is rare. A good tip for handling many soldiers with only a gun is to use the guard vision. Guard vision will place your shooting range in front of you. If an enemy soldier goes onto that vision range, the enemy will be killed immediately. This is useful method and it does not not matter how skilled you are at shooting in the game. The CPU will always do better.
-Use the following trick when you have the diver on a mission with an enemy that you get behind to throw the knife at. The game's line of sight feature works as follows. When you lay in the light area you will not be spotted, but if you stand up a blue sight will appear, indicating that your enemy does not know who you are. The longer you stand up, the more suspicious he will become. However, if you lay down in the darker area the same thing will happen. Use the diver then take out his knife. After that, go up to the enemy then lay down in the darker area and crawl towards him. The blue sight will appear, but do not worry as he will not notice you for awhile. Once you are in range, throw the knife. If done correctly, you will kill him while remaining safe.