Back Against The Wall
When on multiplayer and playing the Sword Base location, there is a spot where you can't be hit easily, but can snipe most of the place. You'll need either a sprint or dodge armor ability. Go to the ground floor and approach the gravity lift in the center from the window lit side. Take a running start at it and let it fling you into the air. If done with enough of a run, it should fling you onto a ledge high on the wall. Run all the way to the upper side of it and there should be a groove you can stand in where you can't be hit easily.
More tricks, tips and cheats for this game are right here - Halo: Reach cheats
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