At the start of the level you don't have a weapon, so jump down from the building and take cover against the wall. There's 5 or 6 soldiers in a small group, including a Dragan. If you wait a few moments one of the soldiers will walk over to the wall and you'll be able to stealth kill him and pick up his pistol. You can either shoot the other soldiers to death, but it may be easier to just stealth kill them all.
Once that's done make your way up the ladder and to the roof of the building, you'll see a door. Attempting to kick it open won't work because it's been barred shut from the other side. Look for the sign-post in the middle of the square and jump onto it, then just shoot the wooden panels that are barring the door. Once you've done this, you will be attacked by 3 waves of soldiers.
With each wave you'll have to alter the direction you're facing on the sign-post depending on where the soldiers are. Also, you have infinite ammo at this point, so fire away. If you shoot the wooden boxes enough times they will break and you'll be able to kill any soldiers that were hiding behind them.
Once you've killed all the soldiers Elena will lower a bridge for you. Jump onto the bridge and make your way across picking up any ammo lying around as you go. Climb up the ladder to the next area.
After a short cutscene Elena will jump to the next building across, do not follow Elena as you will fall to your death. To your left there should be a rope which you can slide down. Once you've done that, there should be a cart next to a door. Push the cart out of the way and go through the door to enter the warehouse. On the left there's a single train car on a ramp, jump across the other train cars to reach it. Inside the train car there's a lever at the front. Pull this lever.
The final part is the hardest, you need to defend yourself against the giant group of soldiers that you've just interrupted. All you need to do here is live long enough for Elena to arrive with a 4x4, which is easier said than done. You'll be up against 10 or so normal soldiers, as well as 4 snipers and 2 Heavy Weapons Troops which arrive about half way through. There's plenty of guns in the train car though, including a few sniper rifles. The best way to get through this last part is to just stay in cover and blindfire kill any soldiers that get too close, and use the sniper rifles to kill any snipers that appear.
It's possible to simply wait in the train car for Elena, rather than killing everyone, as only one or two troops will approach the train. It's also possible to jump outside of the train car.