When you start the fourth mission "Upriver" you will have to take down a guard on a sniping post, just sneak by him by crouching or walking quietly, when you are close hit him with your knife.
Now to the sniping part, it is pretty easy, you will have to shoot some guardians before you can regroup with your units. Shoot ONLY when you hear a thunder sound, you can predict it coming by noticing the flash, so your shots won't be heard.
First take out the one on the left pier when thunder strikes, next aim right where you can see two other guardians on two piers. The thunder will strike, and if you are fast enough you can take them both, otherwise wait until the thunder strikes again. Now reload.
Aim a little bit up, a guy will come down the stairs, hit him when thunder strikes. Now the tricky part, you will have to take out two guardians at the same time, ONE SHOT KILL when thunder strikes. This can be hard since the alarm goes on when gunshots are heard. You are done sniping now, go down there and regroup.