Sniping is a tactic used in combat, when one uses a sniper rifle, usually a wall-break, and snipes at people from a safe distance. Usually snipers are deadly and deal high damage, but chances are they'll be toast if you sneak up on them. Also there's one technique that is similar to sniping, but it's much faster, called Quick-scoping.
Use a sniper rifle that deals high damage but doesn't shoot a laser trail, as your location will be revealed.
Sometimes, one can use a Primary with a 4x zoom as a makeshift sniper if one is accurate enough. If you do this, try to burst fire instead of holding down the fire button, due to the fact that many automatic weapons have recoil.
Turn on auto-shoot. There is a high chance you will miss when firing. When you have auto-shoot on, you will automatically fire when you crosshairs rest on a target, but will stop firing when your target moves.
A sniper is more of a defensive weapon than an offensive, but a highly trained sniper like the Elephant Hunter can be used as an offensive weapon (through quick-scoping).
Sneak up on them with a high fire rate, automatic weapon such as the Automatic Peacemaker, as this will catch them off guard. Be warned, as experienced players can still eliminate you without a sniper scope.
Being a sniper yourself can be handy.
Use the Stealth Bracelet so he will have a hard time detecting you.
Never stop moving. Since most sniper rifles have a slow firing rate and reload, you'll have lots of chances to eliminate them.
If they themselves are only using a sniper rifle, ambush them with a shotgun like the Ultimatum or a defensive melee.