Phase 1 Clearing the lower field
All the guard at the lower field must be killed, though the patrols should stay alive; killing them will araise the alarm. Have look at this map. When the mission starts, immediately run with your green beret to the position where the decoy is (yellow encircled), release your decoy and run the point where he stands now (green encircled). When the red encircled guard turns around, he will notice your foot prints and will run to you to investigate. When he just turns around the corner of the barrels, knife him. If you do this well, he might perhaps say something like `halt', but you should not allow him to raise the alarm.
Now, activate your decoy, so the blue encircled guard will come to it and knife him.
The two other guards can also be killed easily; just look at this map. If you make a foot prints track in a triangle, the red encircled, will head for the lowest angle of the triangle first, so you have a little time to knife him. Then, knife the black encircled one (take care for the scan range!) Now, dig your green beret in at the black X [f]. Run with your spy towards the black Y and crouch if you are in the scan range. (Remember! guards who will see you running, will immediately start to shoot). Stand up, and if the guard has you under shot, crouch and hide behind the building. This will cause the guard to run to where he saw you. When is has walked over your green beret, dig out your green beret [1, right click on his position] and knife the guard. Be sure that his body is out of sight of the patrol. Be also sure that you don't stand up with your spy when the patrol is too close to the guard; they will hear him saying `halt' and take a look by their self.
Now you can take the uniform with your spy and kill the other guards, except the three that stand around the cable car. That means, the two groups: one consisting of two and one consisting of three guards. If you kill one guard of a group (with a lethal injection) when the other have turned around, your spy will not be unmasked. When the guard who turned around sees the dead body later on, he will go to it, so you can kill him too. Have a look here for the group with two guards and here for the group with three guards.
Killing the three guards near the cable car can be done with the `under shot trick'; take a look here. Just go with your green beret to the most northern one (crouch the last few yards) and stand up. If you wait, he will hear the other guard say `halt' and turn around to take you under shot himself. If you stand too close to him, he will start to shoot, so take care. Now quickly give the three guards an injection (you know which one...:-) and hide the bodies between the two buildings. This trick requires a little timing, because of the two patrols.
Phase 2 Finishing touch
Send your spy into the cable car, so he will go upwards. Kill the guard at the platform near the cable car. If you are lucky, the patrol walks near the radar, heading northwards. In that case, you can attract the guard that stands west of the barracks by moving the body to the east side of the platform. Remember, that if he sees you while carrying the body, quickly crouch and put on your uniform [u]. When the guards sees the dead body, he will run to it, so you can kill him on the spot. If the patrol is not walking at the right spot, just stand the same guard and wait until the patrol walks past you, heading northwards. Quickly give him an injection and hide his body on the platform; take a look here.
Now this guard being dead, it is time to bring your green beret up. Go into the cable car with your spy, so it will go down. When it is down, sent your green beret into it, so it will move up. When it is up, quickly unload your green beret and crouch. This requires a good timing of three patrols (two down, one up), but it is not too difficult.
Unload your green beret and crouch (and for to be safe, hide behind the building). Then unload your spy and run towards the east, so you can shoot the barrel which stands near the barracks. When the patrol walks near it, shoot it (and crouch/ put on your uniform if necessary); see here. Though the whole barracks explodes, still one officer manages to get out. Killing him and the two guards you left alive is not difficult, since you can shoot them. When done so, take a barrel with your green beret, put it near the radar, run to a safe distance and shoot at it. Bring your spy and green beret in the autogiro.