At the start of the 4th mission (called “Upriver”) you'll notice that you have to clear out a sniper post. Its only protected by one guard so you can sneak by him by holding the crouch button and then hit him for a silent kill with the knife.
Now that you have the sniping post clear you need to get ready to take a few people out. You'll notice that the weather is rather foul, which can be used to you advantage. You're going to need to take down the guards so you can get back to your unit.
To do this without being detected you need to time the shots so they are masked by the sound of thunder. Just before the thunder hits you will see a flash of lightning. Thats your cue to unleash hell.
The sequence of guards to take out is as follows: Left Pier (1 guard), to the right there are two more on individual piers. These two can be taken in 'one thunder' but I'd recommend waiting it out, its not really worth risking.
Once you've reloaded aim up and you'll see a guy coming down the stairs, take him down as the thunder strikes.
The final part is the hard bit. You have to take down the two remaining Guardians simultaineously. One shot under the cover of thunder.
Once they're dead you should head down to regroup with your unit.
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