Make a character with high luck because sniper rifles have bonus to critical chance and at least 6 per or five if you plan on implants to get the perks sniper and better criticals
Crouch, breath, hold, look, listen, stop, aim, squeeze, fire
(LOL WHAT A NOOB)
First, have ED-E as a companion. You will know about everyone before they even start looking for you.
Otherwise, look out for trouble after every step. Don't press the running button and go do something else, you wanna get the first shot on your opponents.
When you first see your "prey", crouch and pull up your sights. You wanna get a clear shot, and stay hidden while you lead your first shot.
Most enemies die with a headshot, but some you have to go for a leg-shot, because you know they will reach range to hit you.
Stay crouched when you know the place is swarming with enemies, and try using a silenced sniper, instead of the Anti-Material Rifle (assuming you are have a guns char. If you are a Energy Weapons, stick with the Gauss Rifle and nothing will stand after a Sneak attack crit. headshot, assuming you are already maxed out). Better rate of fire, and you go back to hidden in no time.
And remember, the night your best ally.
Oh, and remember to invest in Sneak and critical strike damage bonus perks. You wanna get Sneak attack crits. on your enemies every chance you get, and kill even the toughest enemies with one hit.
Hope it helps.