As you re-enter the TRENCHES from the CHAMBERS OF TULUN, turn left, park the motorcyle anywhere in this starting area, and dismount. Face the large palm tree to the east and look for the gray concrete bracket jutting out from the building to the left of the tree.
Use the crossbow or revolver + laser sight to target this bracket.
One normal arrow (or pistol bullet using the laser sight bug) will destroy it. Later, you'll climb along the crevice where the bracket was.
Now, go under the pointed archway to the north and up the wide staircase.
At the top of these stairs are alcoves on the left and right. Climb into the alcove on the left, turn left, and drop into a shallow depression to find a large medipack.
Climb back up, cross to the other side of the street, climb into the alcove there...
...and turn around.
There's a sniper on the opposite rooftop (above where you got the medipack). You can take him out from here or wait until you get up on the ROOFTOPS.