Like the title of chapter says, you'll just need to keep moving. There are no treasures in this level, so there's really nothing to stop for. Follow the girls when you can and try not let Jeff take too much damage. As you're moving, you can still aim and shoot, but only with the handgun.
Get Jeff to safety[edit]
Head down the steps and into the doorway on the left. Follow Chloe around the debris in this room to the right and out the other side. Move straight ahead, and take out the guy that attacks on the left when you cross in front of an alleyway. Shortly after that, Chloe gets attacked by an enemy and you'll need to take him out as well. Right on his heels two more guys will approach from the front at the top of the stairs. Keep shooting, but keep moving toward them as well. When you get to the top of the stairs and head down the narrow alley, a truck with a machine gun will pull up and start blasting you. Duck through the red door on the left that Elena kicks open for you, then follow her to the other side of this new area where she gets blown back by an RPG. Take out the RPG gunner up on the platform on the building ahead of you, then continue following the girls.
Unfortunately the truck shows back up, cutting you off at the end of the next alleyway, so head into the building on your right as it's Chloe's turn to bear the brunt of an explosion. Head out the other side of the building back onto the street, then move around the concrete barricade on your right and into the door beyond it. This leads to a stairwell you'll take up to a dead end, where Lazarevic and his men catch up to you. After escaping them, you'll need to keep running while they all hunt you down.
Escape your captors[edit]
You regain control running away from the apartment you were in, towards the camera. The camera will zoom out to let you see where you're going, and the first obstacle is a metal railing you'll need to jump over. Keep running toward the camera, even when it switches perspective until you take a big jump off a ledge for a slow motion view of the helicopter above you. After this small cinematic, the camera will be behind you again and you can run normally. As soon as this occurs, a truck slams into the alley ahead of you, so take an immediate left. At the end of this side street, jump out to a blue pole on the right, then swing jump over to the climbable bricks in the wall ahead of you. When you get to the top, press 
to have Elena jump over so you can pull her up. From there run down the alley and jump off the next ledge, where you'll find a hiding spot in a destroyed building.