Tranq the guy in front of you on the bridge, hold him up, use the stun rod, just dispose of him non-lethally. You can then either use a box to climb up to the other side of the bridge, or tranq the enemies down below and run up the stairs and into the next area. In the second area, stick to the left (next to the rock walls) and tranq the enemy on the left side of the truck when he appears. Either wait for the guy on the other side of the truck to turn around towards where you came in originally or tranq him, and move on. In the last area, tranq the guy on the ground in front of you a ways (middle of the area in front of you), then take care of the guy at the bottom of the ladder (tranq, CQC, stun rod, whatever you want, just do it non-lethally). Climb up the ladder, then go to the wall behind the prisoner and disarm the two claymores (stick to the far left, then come up behind the claymores), and you're done. Finish it fast enough and you'll get an S-rank.