Set up Lone Wolves and keep playing until you get a match on Narrows, preferably King of the Hill. Head over to the man cannon and pick up the Needler. Then, simply go back and forth between the cannons while unloading on people on the bridge. Only focus on one person at a time, though, and you should be able to kill him. You don't even have to worry about ammo because once you land it will refill since you are stepping on another Needler.
After you land, you may be able to get a clear view of a hill location to your left or right, depending on where it is at the moment. You will have to jump, facing the snipe spawn, to see over the ledge. You may get lucky if you throw a few grenades and get some shots in there, but do not do anything too risky. Going back and forth between the man cannons hardly ever draws attention because people want to get to the hill and you are only slowly killing people off.
This picture from haloforever.com shows where the Needler if you do not already know. It should be the exact same on the other side.
Hope it helps.