Never camp out in one spot, no matter how good you think it is. It also makes you harder to hit. -Always aim for the head. A headshot is an automatic one-shot kill.
-Check your motion sensor. You'll spend a lot of time trying to find people so it's good to zoom out every once in awhile to see if anyone's coming. Try doing this after every couple of shots or so.
-Unless you're incredibly skilled with the sniper, please do not try to use the sniper in every situation(i. e. in close and the enemy has a shotgun).
-Take good shots only. This kind of goes with the point above. Do not jump of a building and try to no-scope someone across the map. Take shots that you know have a good chance of hitting their target.
-Aim with the left thumbstick. It keeps the targeting reticule steadier. Use the right thumbstick only for minute adjustments to get that needed headshot.
For more Halo 2 tips, go to http://www.halo-insider.com/halo2tips.
