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 as fast as possible. 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 which indicates 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. Next, 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.