Go 2 the barrels in the corner where you can see the sniping building. Jump on the barrels on the wall.

Then sprint and jump on the second pair of barrels. Then jump on the red bar.

Crouch and walk as far as you can to the wall. Press A and you should float right up. If it doesn't work, crouch again, walk up to the wall as far as you can and try it until it works.

I've done this on many different occasion with sucessful results.