I used to do this mission by killing everybody else BEFORE I took out the two guards standing by the transmission equipment. Then, I'd come back and kill them, use the equipment, then make a bee-line straight for the end. But, that took me about 12-15 minutes. When I saw that you had to do it under 6 minutes I didn't know what to think. So, I did it this way and actually ended up OVER THE END FENCE BEFORE the helicopter even landed. Here's how you do it.
When the mission starts, immediately run to where the 3 guards are standing. Stop before you get to the edge of the tree line else the sniper on the backside of the truck turns to investigate. Crouchwalk up to that sniper and stab him with your knife. Breaking his neck takes too much time and there are two patrols coming straight for your position. Grab his sniper rifle and lay flat.
Crawl to a position just to the left of where you are at so that you can fire a single bullet through both of the guards on the other side of the truck. Shoot them both with a single shot. Get up and RUN to the equipment and use it. Taking out the two guards by the equipment triggers an incoming helicopter to drop off a whole bunch of guys between you and the endpoint.
Once you've used the equipment immediately RUN along the treeline to your left and head straight down the center. Be careful because there is a patrol over the hillside to your left AND there is another patrol just coming over the hill on your right heading down to the transmission equipment.
Keep running! When you leave the treeline on the left, head straight out across the first field going straight for the center of the woodline up ahead. While you are running, a helicopter will fly over your position from behind you and the machine-gunner in the copter will start shooting at you. So, when you hear the helicopter coming up from behind you and bullets richocheting all over the place, dive to a crawl and get our your sniper rifle. Drill the guard ahead off to your left.
When the helicopter flies off ahead of you, get back up and start running again straight for the middle of the woodline ahead. Don't worry about the other snipers along the outer edges of the woods. Keep running until you reach those woods. Be careful because there is an Armored Vehicle coming up the woodline to your right.
At this point, you've made it into the woods and you are behind both patrols who are now searching the first area trying to figure out who wasted their 3 guys guarding the transmission equipment. And you've gotten behind the line of snipers. Keep moving!
When you exit out the other side of the woods, you'll see some bushes in the the field ahead. You'll also see the chopper hovering past those bushes. Keep running straight through those bushes and use them to block the machine-gunner from seeing you. Keep moving until you come out the other side of those bushes and have to cross the last open area. Be careful here because there is an Armored Vehicle slowly coming over the hill to your right. And, there is one approaching from the road off to your left. DON'T RUN WITH THE CHOPPER HOVING RIGHT THERE! The machine gunner will drill you clean through! Run a bit past the bushes and then dive to a crawl.
Keep crawling in a zig-zag pattern. Eventually, the chopper will hover forward a bit then start to land. When the chopper starts to descend, get up and start running again. Run straight through the bushes up ahead which are right in front of the entrance to the exit area.
You have to get into the entrance of the exit area BEFORE the patrol standing in the bushes off to your RIGHT starts patrolling. They only start patrolling when the chopper lands. Keep moving!
By now, you're heading straight down the leftside of the entrance to the exit-area and the chopper has landed. The patrol in the bushes off to the right of the entrance area has started running around crazy. Keep moving!
Run down the entrance. Follow it to the left then it veers right. Cut diagonally across and just as you turn right, run along the right-side of the rope. Be sure to keep those trees up ahead directly between you and the sniper in the tower in front of you. Part way down the rope-line, dive to the ground and quickly pull out your sniper rifle. Snipe the guard in the tower clean through the chest. Get up and toss your rifle. You won't need it anymore.
Run straight to the fence by the bottom of the tower. Climb the fence! If you've done everything fast enough, you'll actually get over the fence BEFORE your rescue helicopter lands! Mission accomplished.
Lots of running in this mission but the trick is to keep moving even when you are crawling. And, make sure that you crawl in a zig-zag pattern while the machine gunner in the chopper has got you in his sights. You have to get into the entrance to the very last area BEFORE that chopper lands else the patrol on the right of the entrance to that last area will find you before you can snipe the guy in the tower and get over the fence.
One last trick.. You'll run much faster if you're carrying your knife.
I did this mission in 4:24 without being seen and 100% accuracy.