When I saw that you had to complete this mission in under 10:00 minutes, I thought that's impossible to go from the jail cell, through the tunnels, kill the guards, and open the gates in that short of time! After piecing together how it could be done, I finally completed it in under the time limit. Here's how you do it.
When the mission starts, you're standing in the cell. Immediately lay flat otherwise the guard hears you. Crawl up behind the guard, then crouch behind him and slice him with your knife. Keep crouch walking straight and take a left down the hallway to where the other guard is so you can pick up your stuff. Don't bother stopping for the camera. Keep crouchwalking straight into the guard with your knife and slice him up too. Keep moving straight to the end of the table and grab your stuff. Turn right around and crouchwalk straight back down the hallway. Don't worry about the camera. You can crouch walk out of its range before it goes off!
Go back down the hallway and continue straight towards the opposite door. Pick the lock. Crouch walk straight up the stairs. Stop and lean over the edge of the stair to make sure the camera is facing away. Walk up the rest of the stairs and straight to Priboi's cell. As soon as that objective is complete turn right around and head straight for the door right there across the hall. Open the door then crouch walk the rest of the way into that room to pick up your silencer pistol. You'll need it! Turn right around, go straight out the door, turn right and fall right off the edge of the floor to the lower level. The camera won't see you long enough to sound the alarm.
Crouch walk back to where you first picked up your stuff and get out your silenced pistol. Keep aiming upward a bit and keep crouchwalking right to the bottom of the stairs and shoot out that camera with 2 quick shots. Crawl partway up the stairs and stop. You have to lean upward and as soon as you see the guard come out from around the corner, crawl to the door! Pick the lock and crawl out as fast as you can. If you wait too long for the guard on his route then he'll see you while you're still picking the lock. The guard ONLY looks at the door just as he returns from his route and comes from around the corner. After that he doesn't look. So make your move and make it quick!
Get outside the door and out of view. Crouchwalk diagonally across to a position behind the boxes in front of the hole in the fence. Nobody sees you. Check your map computer and make sure that the guard patrolling back and forth in the outer courtyard is NOT right there. When he turns away, then crawl out through the fence and keep moving around until you get up behind the door of the shack on the other side of the fence. Crouch and pick the lock. Go in and slide down the ladder.
When you get to the bottom of the ladder, get out your silenced pistol and run 3/4 of the way along the rightside of the sewer then dive to a crawl and keep looking straight forward through that metal rack up ahead. Keep crawling and as soon as you see a guard appear on the other side of that rack, drill him through the side with 3 quick shots. This causes the other guard facing away from you to turn around. Pluck him with one shot clean through his head. Remember, get close enough to the metal rack but not too close else the second guard will see you when he turns around.
Now, get up and cross over the sewage to the walkway on the other side (where the guard was standing) and turn right.. Don't run else the guards above ground will start running around crazy. Turn left then crouchwalk part of the way then crawl. There is a guard to your right just around the corner. Pluck him with a single headshot. Get up and start walking again.
When you turn the last corner, lay flat and crawl with your binoculars on zoom. When you've got that last guard's head zoomed in your view, pull out your silenced pistol and pluck him with a clean headshot. He goes down.
Get up and reload. Walk the rest of the way to the ladder. Climb the ladder quickly about 1/3 the way up then climb slowly. If you climb quickly ALL the way up the ladder then the guards in the nearby building come out to investigate.
When you get to the top of that ladder, crouchwalk out the door and use your heat-vision goggles to estimate the guard's position in the tower. Fire straight through the wood with 2 or 3 quick shots. You should hear him collapse.
Get up and WALK around the wall and straight for the corner of the building. When you turn the corner, crouchwalk under the camera and follow the wall all the way around until you get behind the boxes on the other side. Don't take the cable because when you land, it sometimes causes them to come out to investigate and you don't want anybody out in the courtyard who doesn't have to be there already. You've got enough guys to kill.
RUN along the back wall and then cut diagonally towards the side of the building with the computer inside. Crouchwalk the rest of the way, right under the camer and straight into the door. Disable the alarm, and crouchwalk straight out of there.
Keep crouchwalking to the building above the main gate. Go right of the stairs and straight through the door. Follow the wall around to your RIGHT and get into the far corner directly behind the two guards in the backroom. Here's what you have to do.. Pluck the guard by the door in the head then immediately drill the other guard in the back 2 or 3 times. Occassionally, I've missed this second guard who turns and spots me, fires a few rounds, then runs out the door to turn on the alarm. Don't miss this second guard!
Once you've taken care of both these guys, crouchwalk back into the other room and then HIT THE BUTTON to open the outer gate. Before climbing the ladder, quickly use your heat-vision goggles to check to make sure the sniper on the roof is not looking your direction. Climb the ladder slowly. Get on the roof then crouch and pluck this guy in the head with a single shot.
Crouchwalk and pick up his rifle then pick up the second sniper rifle. RELOAD the sniper rifle. Here's what you have to do next.
Right at the spot where you picked up the second rifle (leaning against the wall), crouch jump onto the wall there and lay flat facing the corner area where the last truck is parked. Look through your scope and you'll see 3 guards with one standing partially on the other side of the truck. Zoom in on the guard's head just peering over the edge of the truck and pluck him. Then, immediately pluck the guy facing your direction. Immediately pluck the last guy! If the last guy starts running the pluck him quick! Immediately after you've plucked the last guy, drill the sniper in the tower. RELOAD!
The two guards by the alarm switch in the nearby area will be looking upward because they heard the bullets flying overhead. Take out the guard closest to the alarm switch then immediately take out the other guy before he starts running around crazy. This second guy moves quick so drill him fast!
Now.. RELOAD! Get up and crouch jump onto the boxes. You'll need this new location to have a clear shot at the guys under the tower diagonally to your left and you'll need a clean shot at the guys trying to turn on the alarm by the gate where Priboi exits.
Zoom in with your scope on the alarm box by the gate. Wait for the guard patrolling back and forth to pause by the alarm switch. Get ready! Drill him clean through with a headshot and then turn and take out the guard to the right of that gate who is closest to the alarm switch. You should be able to get him. Try for the second guy too! Too often other guard runs to the alarm switch. If he does, you have to quickly re-aim right above the alarm switch and snipe that second guy BEFORE he hits the alarm. Sometimes both guards start running and you have to take both of them out BEFORE they each hit the alarm. If you've done it right then nobody comes running and the alarm doesn't go off. If the 3 guards from the building right there come out then you have to take them out too. Two are easy because they also run to the alarm but don't turn it on. Sometimes they come out and sometimes they don't. RELOAD!
Now, killing the guys by the alarm switch causes the two guards by the truck near the tower off to your left to run around. This is the perfect opportunity to take out the guard in the tower. If you shoot the tower guy with those guys watching, for some reason, all the other guys in the nearby building come out. Take out the tower guard and you have to take out those two guards below the tower whenever they cross into your sites. Keep firing and take them out!
Once these guys are out, check your map computer. The guy standing on by the shack you went down the ladder won't do anything. Leave him alone! If you pluck him then guards inside come running out. He won't see Priboi and he won't see you!
Crouchwalk straight off the edge of the building and land on top of the chainlink fence below THEN drop into the little area leading to the main gate. If you drop all the way to the ground, you'll get hurt. Get up and walk to the gate switch. Open the gate, walk around to the other gate switch. Don't run else the guards in the building come out! Activate that switch too. Now, turn and RUN straight towards the other switch by the gate where Priboi exits. Hit the switch and run straight back to the door in the fence.
Unlock the door in the fence. Activating the last gate switch causes Priboi to leave his cell. Go through the fence-door and get back onto the building your were doing all the sniping from. Stand and wait from Priboi to make it to the truck. Mission Accomplished!
I finally did this mission in 9:16 without being seen and 100% accuracy, 100% health.