St. Petersburg revisited is a pretty difficult level because you start with no weapons except your trusted fibre wire, meaning that you can't actually shoot your way through this. Well, the first time, you can bring all the weapons you can carry the second time around.
It is advised to try this on your own without this walkthrough at least one or twice to fully appreciate what the developers tried to do here.
How they want you to do it
-Pick up the sniper rifle from locker #137. Note that it has no bullets.
-Make your way to the second pick-up point to pick up the bullets. If this proves to be difficult, drop the sniper and the guards won't bother you. Don't try to go in past the two guards (with your sniper rifle), there won't be a gap in their patrol for you to sneak through. Instead, go back around and enter the building from the front. Stick to the sidewalk of the Varosnu square for uhm... no real reason.
-Enter the Pushkin building and get to the same window as the last time. You can see a figure that is totally Sergei and not some cardboard cut-out looking straight at you with a nasty smirk on your face. Shoot him.
-Hm, definately a real person Sergei doesn't seem to budge. Maybe you've been hitting the window stil every time? Better go check it out close and personal then.
-Enter the main building and... what's that strange tingling sensation in your brain? Did you just have an idea? Hm, no. That's a bullet. Jup, definately a bullet...
-As your consciousness fades, you look at your bullets and notice they are blanks. Hm... You're starting to feel like this was all a set-up. With your dying breath you aim your sniper rifle at pretty much anyone and fire, only for the bullets to do absolutely nothing at all.
Silent assassin rating
Using your meta-powers to somehow reverse time with the knowledge that this is a set-up, you stand in the metro again.
-Ignore the sniper rifle, it is completely useless. Either make your way to the Varosnu square through the sewers or the streets. Without a sniper rifle, no one will be bothered by your presence, not even the guards marked 'enemy' on the map.
-Don't enter the square just yet. That sniper fire came from Agent 17, a clone you missed in the previous game. And he's got very keen senses. If you can see him, even while needing your binoculars, he can see you too.
-Avoid his gaze and walk (not run, you absolutely mustn't run even a little bit once you get to the square!) into the building once he cannot see you. This might be hard to determine considering he doesn't show on your map. From what I can tell, he has a bit of a blind spot for the left sidewalk from the southern road, probably to allow you to get to the Pushkin building.
-Walk into the building and up the stairs. Sneak up on Agent 17 and strangle him with the fibre wire. Note: If you ran at any point while in the square, he heard you and is alarmed. It is now IMPOSSIBLE to sneak up on him even with ctrl sneaking. If you did this, either restart or try alternative way 2) or 3) if you don't want to redo everything.
-Take his uniform and W2000 Rifle and run away. If you handle fast (and skipped the cutscene), you can make it all the way to a sewer entrance before the guards find his body and start looking for 'a suspicious clone'. There are a few guards stationed in the sewers thanks to teleportation formations or something (There is no outrunning them or the guards in front of the metro entrance) but there is a path through them that will allow you to get through unopposed. Don't go through the streets, there is no getting through the guards at the metro entrance.
-Get to the metro and leave. Optionally, you can pick up the empty sniper rifle from locker #137 on your way. Sure is a good thing they didn't station those guards right in front of the subway carts, isn't it?
There are three alternative ways to complete this level, only the first requires you to complete it at least once before.
1) Select as much guns as you can carry before starting the mission and go in guns blazing!
2) It is possible (though untested because it's rather tedious) to sneak up on the most southern guard near the second agency pick-up and strangle him with the fibre wire while he's standing near the Ural truck. Take his gun and shoot Agent 17 with that.
3) Run up to Agent 17 and pick up the W2000 rifle after he saw you. This is possible because he'll drop it and run to the entrance after hearing you run on the square before returning to his sniping spot. When he hasn't noticed you yet, he'll still be holding it, but now he'll be holding a Deagle (or revolver? I forgot) instead. Note: You can't collect this handgun when you strangle him.
Take the rifle and shoot him with your one bullet in the brief window of opportunity before he shoots you with his much better and more appropriate handgun. Take the handgun and prepare for the guards to come up so you can shoot them too. Getting past them unseen will be tricky, you probably won't even get enough time to put on 17's disguise (although maybe you will, I didn't skip the cutscene).
Shoot your way through or, if possible, sneak using the same disguised strategy as mentioned in 'Silent assassin rating' to get to the exit.