My rules of survival, for a sniper :
1-Stay undetected or die trying. Take the time to aim for the head as less bullet shot will limit your risks of being spotted
2-Smart use of cannon fodders : your friends in first line will hide your fire as enemies will focus on them, you'll be harder to detect. They can also help you detecting enemy snipers
3-The bait : if enemies spot you but don't kill you, they will focus on your last known location. Take a new location with your former one in sight, you'll have some easy targets
4-Dissuasion or slaughter ? : shooting enemies once you see them will force their squad to take cover. Waiting all are a bit closer will allow you kill many. Sometimes waiting a bit to decimate a full squad is a worthy choice