At the top of the apartment complex, on the southern side of the south island, Sanchez awaits. A fast way to reach her is by climbing the windows of the blue, western building to reach the top. Thankfully there aren't very many armed guards at the top. She shouldn't pose much of a threat. Due to the rooftop's close quarters, your melee attack are quite useful. Also, shoot explosive barrels if Sanchez is anywhere near them to inflict heavy damage.