Instructions
Pick up the sniper rifle in the beginning of the area and equip it. Crouch down and move forward. Aim with the sniper rifle to use its scope and look forward to see two enemies. Shoot both of them with the sniper rifle.
Move forward until you get just around the corner to the next part of the alley. Don't move too far out into the open -- move just far enough so you can see around the corner. Aim with the scope again to see a man holding a rocket launcher and shoot him.
Look toward the top right of the screen to see an enemy carrying an AK-47. Take aim and shoot him before continuing down the alley. Switch your weapon to an AK-47 and take a left when you are able to. Follow the linear path into the building while taking out any enemies.
Go upstairs and take out the enemy with the shotgun. Explore the upper floor of the building to find ammo, grenades and enemy intel. When you are done replenishing your supplies, go downstairs and head into the basement. Take out the enemies here and follow the linear path out of the basement to the next area of Sniper Alley.
Run forward as soon as you get out into the open and look for a recessed area on the right with three men facing away from you. Kill all three of them and pick up the rocket launcher. Go back to the main alleyway and fire a rocket into the building on the horizon. There are several enemies in here, so this is a good chance for a preemptive strike.
Run to the end of the alley and take out the two enemies with your assault rifle. Switch back to the rocket launcher and fire a round into the building to the right. Walk down the path toward the building and aim to the building to the right in the distance, and then fire another rocket toward the third floor to take out another group of enemies.
Go into the building and take out any remaining enemies. Collect the intel, ammo, additional rockets and health packs, then exit the building. Take the first left when you're back in the alley. This area looks like the ruins of an old tunnel. Run down the tunnel and take a left to take out a group of enemies. Turn around and lob grenades toward the enemies on the small hill across from you.
Go back toward the tunnel, but run straight forward to get to a new area. Follow the linear path to get to a large battlefield surrounded by several buildings. Take out the enemies near the opening to the battlefield, then kill as many enemies as you can with your rocket launcher before running into the fray. When you get to the other side of the battlefield, the "Cross Sniper Alley" objective is complete.