THE GUARDS NEAR THE OTHER SIDE OF THE BRIDGE
There are 6 guys in total guarding the other side of the bridge ? 4 inside the barracks, and another 2 outside it (standing near the alarm switch). The 2 guys outside are very easy to kill. First of all, you need to hide behind those nearby crates. It may seem a bit difficult at first (since the crates can?t completely hide you from both guys when approaching), but as you get closer to them, you?ll notice that the upper crate can indeed block the guards' view (see this pic), so if you approach them in correct angle + in CRAWL mode, they won?t spot you at all.
Once you?re near the crates, switch to CROUCH-WALK mode and activate the Thermal vision. Hug the crates, and then position yourself near its left side as close as you can in order to not be seen by any of those 2 guys (see this pic). Now deactivate the thermal vision, which will make both guys staring in your direction. Once that?s done, don?t waste any time! Quickly go around the right side of the crates (using strafe keys, of course) and then kill both guys by twisting their necks. If you didn't waste any time (that is, if your forward & strafe key were already pressed during that cutscene when killing the first guy), they won?t have a chance to notice something - see this pic.
Now destroy the nearby camera with your silencer and then pay attention to the 4 guys inside the barracks. You can kill them in 2 ways:
1. By luring them out via shooting to their legs, then finishing them with a grenade & your silencer
2. By simply shooting them through the doors with the AK47 (by doing this, you won?t be able to collect the AK47s and grenades, which they occasionally leave).
If you want to kill them outside the barracks, just let them open the bridge gate first (you can?t open it from where you?re standing now) and THEN do the killing.
In case you want to just shred them with AK47 fire before they can come out to investigate, you?ll have to jump the fence and then open the bridge gate from the other side. The trick is to jump on those crates located near the bridge and then do sort of a ?double jump?. That means you?ll first jump on the fence, and then immediately make one more jump to fall through it (see this pic). It?s a bit tough, but it?s possible. Anyway, if you want to use this 2nd option, the trick is to once again activate the Thermal vision and then aim at the 2 guys who are in line. But contrary to the 1st approach (where your goal was to just injure them), this time you?re going to kill them, so point at their chests (see this pic). Once they?re in your aim, fire cca 5 shots, which will usually be more than enough to kill both guys. Immediately after you fire these shots, activate the Thermal vision. Usually, one of the guys will try to throw a smoke grenade (whilst the other will do nothing), so use this brief moment to point exactly at his body and then fire another couple of shots towards his direction + another salvo towards the direction of his buddy. Since your shots leave marks on the doors, it?s always easy to memorize how far to the left or to the right the given guy is positioned from these marks, so you should have absolutely no problems to aim properly and then kill the remaining guys before they can open the front doors.
THE LAST 4 ENEMIES
Now all the guys in the barracks are dead, cross the bridge and then do the same with the guys in the barracks located on the other side of the river (if you haven?t killed them yet). If you?ve already killed them before, just grab all the AK47s from their dead bodies, and then pay attention to the 4 enemies patrolling the area near the 1st factory building. Once again, there are various ways to kill them. You can try to throw some grenades, or shoot them with your silenced pistol, but my favourite approach is to go inside that factory building and then shoot them through the wall with your AK47 as they approach the southwestern corner of the building (see this pic). Just hug the wall and then use the thermal vision to determine the guards? position. Usually when you fire a shot, they?ll stop walking for a while, so use this time to make a good aim, then fire a couple of shots (the marks on the wall will once again help you with the aiming). Once you kill the first guy, they?ll usually start running along that wall, so just follow them and then kill them one by one.
Once these 4 guys are dead, there?s no one enemy left for you to kill in this level, so go back all the way to that bunker-like building at the other side of the river and then use the lift to enter the mines (hit the 2nd switch). Mission 1 accomplished!