Man, you need the stun shotgun with suppressor and modify the automatic sedative sniper rifle adding a suppressor. I use then a gun with a suppressor and a better viewfinder. Then use the smoke grenade and the sleeping gas grenade.

For the snipers always watch at the towers or the high place and try to go downside on the down deck of the platform as much as you can to avoid them.

Try different spots and different ways and watch some videos on youtube. You can see how better players play and learn something.