1. Neutralize all alarms: hard 
2. Install covert modem: medium, hard 
3. Intercept data backup: hard 
4. Bungee jump from platform: easy, medium, hard 

Around the first corner, you'll see a soldier on your left. Your default weapon here is the silencer, aka the pp7 (silenced); use it to shoot him in the head. Grab his weapon, the KF7 Soviet, an loud automatic rifle. Now directly in front of you is a guard tower with a few men around it and in it. Tag them all. 

Head to the tower, but go around the left side (not the right side), or you will notify soldiers in the tunnel on the right. At the top is the Sniper Rifle. Look through it, press R, and adjust the zoom lens with the top C button. In the tunnel are two guards. Pick them off. 

Head into the tunnel. After taking the tunnel just past the bend you'll want to move to the right to protect yourself. In the distance you'll see about two close soldiers, peg them and then zoom in to nail the two guys in the bunker. You'll notice a tower on your right (sometimes there's a guard in it) and a large stack of crates on your left (behind it hides a guard). In the easy level there is body armor in the tower. Grab it. To collect more ammunition, blow up the wooden crates near the bunker. Shoot only the wooden boxes. Watch for barrels with skulls and cross bones, which will blow up and hurt you. 

The green truck is waiting at the far end. To its right is a red switch, so you need to press B to it. After it is another switch, press it. A tower is on your right with at least two guards near it. Make sure to hit them, but be sure to quickly zoom in the soldiers in the far distance, the ones nearest the alarm. 

Now dispose of the red alarm on the barrack wall (it looks like a button), and then back up to the red garage door. Open it and be careful of hiding guards as you move inside (there should be at most three). Exit the compound and move onto the next bunker. A set of crates appear just behind the bunker. Blow up the one closest to it, and you'll see a monitor. Attach your covert modem to it (in medium and hard). Head to the fence, shoot at the lock, and press B to push the gates open. 

In the distance you'll see three guard towers. Zoom in and pick off these tower guards with the lovely sniper rifle. There's one guard in each tower. I think it's best to eliminate each guard at each tower, then destroy each alarm, systematically. There is one alarm located in each tower, so just walk through the door and you'll see each one on the wall. 

You can access this downstairs region by going down a ladder near each guard tower door. But the best way is to go to the farthest tower (farthest from your starting point). Enter the tower, walk down the stairs, open the gate, and head left. The upcoming corridor is loaded with guys, so use the leaning technique and your Sniper Rifle. When you've reached the end of the tunnel, open the door and prepare for serious battle, i.e. take a few steps back. Use the KF7 Soviet for massive attack, making sure not blow up the mainframe in the back at the left. Approach the mainframe and press B to intercept data backup, and you're almost done. 

Go back the way you came, and head toward the second guard tower, where, across from it, there is a set of stairs. Walk up and jump off the dam. Mission complete.