This is a tough mission but you can do it in hardmode in under the time limit.
As soon as the mission starts, RUN straight in the direction to where the two guards are standing by your stuff. Get out a grenade. When you get to the edge of the trees, crouch and toss a grenade at the two guards. Don't overtoss the grenade past them. Blow up the two guards then RUN and grab all your stuff and head back in the direction you just came from and straight up the side of the hill. The plan here is to go completely around the entire patrol heading down the mountain! Get up on the hillside and keep running until you get behind their position. If you keep being seen on the hillside then you can crouch walk part of the way. The idea is to grab your stuff and get up around and behind the entire patrol without them seeing you.
Once behind the patrol, run along the hillside around the front of the building. Stay far enough down the edge of the hill so the 2 guards outside the front of that building won't see or hear you.
Keep running around and come up onto the flat area where you can see the two other guys guarding the pass. Quickly lay flat and snipe both guys square in the back. Two shots! Now, get up and run towards their position.
Get any grenades they have and now run towards the back area of that pass and get nearby the last part of the hill so you can see that entire patrol. The trick here is to kill this entire patrol with grenades BEFORE they get back up to the weather station. Crouch down and toss one long grenade, one medium distance grenade, and one short distance grenade and then lay flat. Snipe anybody still standing. Make sure the sniper rifle gets reloaded!
At this point, you've picked up your stuff, gotten past all the bad guys, killed an inner patrol (who will only make your life miserable later) and you're now heading up to the weather station in record time.
Run up the pass to the weather station and get up on the top of the hill to your LEFT. Lay flat on the side of that hill and now you have to pick off the 4 guards patrolling the back area past the helicopter pad without any of them running and sounding the alarm. This requires some accuracy but make sure your rifle is loaded and fire away! You need to save 3 bullets for later. Just make sure that nobody gets back to sound the alarm.
Now, get up and adjust your position on hill so you can see the two scientists in the bunker through the side window. Pick off one then the other. Bang! Bang!
Now, run along the hill behind the bunker. You have to get into a position where you have a clean headshot on the guard patrolling back and forth on the outside platform of the building in front of the bunker The trick here is to wait until that guy is standing still looking off the opposite end of the platform. Take one clean shot straight through the back of his head. Make sure that the bullet will not hit the building else the guards inside will come running out so adjust your position accordingly. One clean shot with the exiting bullet heading off into the air causes him to collapse without him making a sound and nobody comes to investigate.
Now, get up and run along the hill and around the back of the building which has all the boxes out front. Get into a position along the tree line so you can look through your scope right into the middle window on the lower floor. There is a guard inside that you have to take out. You should have him in your scope to get one clean shot through the window into the back of his head. Drill him cleanly and he collapses without a sound.
Run back the way you came and crouch walk into the building with the guard you just drilled. Open the safe and grab the documents. Crouch walk out and head straight toward the second building. Remember, both of these buildings still have guards on the second floor so you can't make a whole lot of noise.
Crouch walk into the second building and crouch walk underneath the camera. Stand up directly underneath the camera and wait for it to turn away from facing the computer. RUN over to the computer and turn off the alarm BEFORE the camera turns back to see you. Once the alarm is off.. crouch walk out of there! You made some noises but if you did it right you didn' do anything which causes anybody to investigate.
Crouch walk out the front door and head straight to the bunker. When you are far enough away from the front door you can run the rest of the distance. Unlock the bunker door. Unlock the two inner doors. Cut off the hinges and grab the EMP Chip. Run back out through all those doors and head straight to the crank-handle. Raise the weather balloon. Mission over!
You can do this mission in under 7 minutes. I finished in 6:37 without being seen, no damage, and 100% accuracy.
Requires accurate grenade tossing and some well placed sniper rounds.. but you can do it!