This is a tough mission to finish under 6:00 minutes with the required accuracy rating. It used to take me 20+ minutes and when I saw that you had to do it under 6 minutes my first reaction was: No Way!!
Here's how you do it.
When you start the mission in the water, immediately swim straight for the shoreline directly in front of you and bypass the guard on the docks altogether. Get up onto the shore and run straight for the side of the building facing you. Then, run down the little alley and turn LEFT down the other alley with the guard 3/4 the way down facing the opposite direction. Run most of the way towards him then quickly lay flat. Then pluck him in the head with one clean shot. He goes down. Then, put two shots into the chest of the sniper on the balcony straight ahead overlooking your position. This is a tough shot but you can take him out if you put two straight into his chest. A headshot at this distance is excellent!
Now, get up, reload, and run straight ahead and lay flat just far enough were you can see the two other guards down the other alley to your right. Shoot one guard in the head then quickly put two shots into the chest of the second guard. If he doesn't go down right away, you have to pick him off without letting any of your bullets fly all the way down to the other end of the alley where the other 3 guards are. Otherwise, they all come running. When you've killed these two guards, check your map computer to make sure that none of the 3 guards heard anything and that nobody is coming to investigate. I've run back to get the Libyan friend and ran back into this alley straight into these 3 guards investigating the area and they took me out before I even knew they were there. So a quick check of your map computer is worth it.
When you are sure nobody is coming to investigate, get up and run back to your Libyan friend. Immediately turn right around and run back to where you were just laying then WALK down the alley and part way up the alley on your right. You have to pluck the guard up on the balconey who is keeping an eye on the sniper guarding the explosive pack. Quickly lay flat and kill him cleanly with a single shot into the back of his head. Otherwise, this guard runs back and forth and he's hard to bring down and your accuracy will drop considerably.
By now, your friend should be coming down the alley near where you are. The trick here is to NOT make too much noise that gets the guard in the computer room running around all crazy because then you have to wait for him to calm down. I've gotten this guy so upset that when the Libyan gave him the password, the guard drilled him right there as soon as the door opened up! So crouch walk to the door with your friend and get into a position next to the box to the right of the door. If there is any doubt about the guard in the computer room then use your heat-vision to check his position.
Wait for the Libyan to give the password and the door opens up. When your friend walks away, lean to check the camera and when the camera is facing away, crouch walk into the room and shoot the guard by the computer in the back of the head. Then, immediately shoot the camera with two quick shots before it sounds the alarm. De-activate the computer's alarm. By now, you've used up about 3:30 of time.
Now, RUN out of there, past your friend, and get to the ladder leading to the sniper with the explosive pack. Climb the ladder quickly and WALK near to the corner then lay flat. Crawl around the corner and carefully put one shot into the side of the sniper's head. This is a tough shot because it's difficult to see his head in the outline of the wall. When he goes down, get up and run and pick up the explosives.
You have to act fast and decisively from here. Crouch jump up onto the rail then lay flat and belly-flop down to the ground. You'll sustain just a bit of damage but that's anticipated.
Once on the ground, RUN to your left down the alley and stay along the righthand wall of the courtyard. Run straight across the entrance into the courtyard, along the inside wall directly to the back of the truck. If you've timed it right, no snipers will see you and the two guards by the alarm switch won't see you either.
Plant the explosives then RUN to your left, past the truck, and along the wall, behind the boxes, and get to a point along the wall just before the entrance to where the two guards are standing. Lay flat along the wall and crawl tightly up against the wall. Make sure that you are laying flat far enough from the entrance so that YOU can't see through the entrance else the guards who will soon come running will see you! Also, make sure that you are not TOO FAR from the entrance that you'll be seen when you get up and start running. Stay just along the wall where you can't see down the alleyway through the entrance opening. The explosives should detonate by now. The explosion causes all hell to break loose so get ready!
Lay there and get out your map computer and watch the two guards by the alarm switch enter the courtyard to investigate. They won't see you up against the wall there. Then, watch as three more guards come running from the rightside of the alley. There's a whole bunch of guards also coming from the other direction so you only have one small window of opportunity to get past these 3 guards and BEFORE the other guards get there!
Here's what you have to do next. Put away your map computer and count the 3 guards as they enter the courtyard to investigate the explosion. The first set of guards by now are over by the truck. A few short seconds after the last of the 3 guards runs through the entrance to investigate, stand up and run as fast as you can straight across the entrance and down the alley. The guards in the courtyard nearby will hear something but by the time they turn around you'll be gone and they won't see you. Occassionally, I've been seen here so it really is a matter of timing and laying close enough to the entrance way.
Run down the alley, turn left and run along the wall of the building on your right. There's a sniper patrolling on the balcony just above that wall and the trick here is to have him hear you and stop to look down to investigate. He won't see you because you are right up against the wall beneath his position. Meanwhile, you've run completely past him, along the wall, around the corner and down the alley alongside the main building where Priboi is being held.
The trick now is to NOT make too much noise running else the two guards on the first floor of the building with Priboi will start running around crazy. Run along the opposite wall and walk part of the way towards the front entrance to that main building. Lay flat before you reach the small open area by the front doors. There's a sniper halfway up on a little balcony to your right overlooking the main entrance. And, there's another one on the roof directly above him. There are also snipers on the roof of the main building but they won't see you.
Now, crawl quickly into position so you can see take a shot at the sniper on the lower balcony. Put one round into his head. This is a tough shot! This causes the roof snipers to start yelling and the first floor guards to start running around but that's okay. This also causes the sniper directly above the one you just took out to crouch down! This is your opportunity to get into the building before he stands back up. Immediately stand and RUN into the building and up the stairs.
Just before you get to the very top of the stairs, dive and crawl the last little bit until you see the guard's head standing there watching the top of the stairs. He won't see you. Peek over the top and give a quick headshot as fast as you can. He goes down. If you miss this shot or take too long then the guards on the first floor are going to come running up the stairs and drill you. Kill this guy and get into Priboi's room FAST! Mission accomplished!
Several times I've stirred up the two guards on the bottom floor of the main building too early (running along the side of the building outside) and one guard came running up the stairs and drilled me from behind just as I was laying flat at the top of the stairs aiming at the other guy's head.
Remember, taking out the outside sniper causes a big commotion because one of the other rooftop snipers starts screaming. But if you're quick enough, you can get up the stairs, kill the guard at the top of the stairs and get into Priboi's room before these 2 guys finish investigating the rooms on the bottom floor before they head up the stairs to check out the second floor rooms. If you stir up these two guys on the first floor too soon, then they'll be finished checking out the first floor rooms and end up heading up the stairs to the second floor BEFORE you can get to Priboi.
Tough mission and requires a bit of luck but you can do it.
I did this mission in 5:48 with 98% health (slight damage from the belly-flop), and 100% accuracy.