Go to http://www.highimpacthalo.org
They are dedicated to explaining Halo 2 (and Halo 1) glitches and out-of-map stuff. They can show you with videos, text descriptions, and screenshots..
I can give you a quick clue though.. Turn on your overshield, use the rocketlauncher, and stand on the "t-bar" on the back of one of the movable "blast shields".. then just shoot the back of the shield, right near the top, and it'll throw you up pretty high in the air.
Although there's a ledge you can get on up there, you're still stuck out there unless you can get someone on the other side of that area to swordcancel... if that makes no sense to you.. have no fear, that website has far more information on that sort of thing.
Have fun!