1 - Reach the sniper's position
Use the slide pipe down and move towards the building where the sniper is.
There are enough cars and containers to hide behind but there is also the issue with the yellow chemical goo. Don't get too close for long.
Cross the river to the right, when the entrance blows up and climb out at the other end.
Use the truck to get over the wall.
You can climb up the house from here. Just get on top of the pile of bricks and climb up using the windows.
On the roof, you can cross the bridge to the other building, take the zipline and climb up where you can reach a ladder.
2 - Find a different way up
Go to the back of the building where the scaffolding structure is. Climb up there until you are facing the building where you can jump over.
Move your way around the building and use the cubed cage to get inside.
Then, go through the door to the roof.