Go to the Greenhouse and look for the stairs that lead to the botom of a mini waterfall.

Once you're at the botom look for a wrecked building and enter the building from the part with a van coming out.

Then turn right and go up the stairs and look for the broken wall. Once you found the wall you will see a bit of it coming out.

Jump on it, turn to either side of the roof and jump on it, then you are there in a very good sniping spot.