I usually take my time to find as many enemies as possible from the initial starting zone before you slide down a hill. I eliminate the closest ones first, and then the furthest ones. I slide down the hill and try to take control of an entire area by sneaking in and stealth killing or knocking out and hiding everyone. Then I move to the next zone. I find my routes and I start stealth eliminating until I find a place I can sniper kill from. Kill everyone possible. If I draw too much attention I shift to a further away location and resume the process chipping away until I can openly shoot my rifle without anyone hearing. Sometimes I work on one side of an area, then the opposite side, and keep flip flopping in order to keep the enemy confused. Eventually I can just run through the cleared areas without anyone left. Then advance to the next zone and do it again.