[Arclight]

Don't miss.

Better to wait for a good shot than start blasting when you spot someone. A missed shot means an alerted enemy, and he's not likely to present you a good shot at that point.

Also, engage targets who are already engaged. People getting shot at tend not to think about the possible sniper on their flank.

Of course that last one doesn't really apply to videogames were everyone is running around like a moron rather than taking cover when engaged, but there you go.