Hmm... I'm sorry I don't know how else to explain it. I just went and played Halo 2, and I have found that my directions were quite accurate. I just did this on Easy, and yes it can be done. My only tips are:
-Are you sure you are in the right place? Its the spot right after the part when you get new marines; when Commander Keyes says "I need a leader who isn't afraid of a little hostile ground fire." (or something like that). Yeah its the wide open fighting ground AFTER YOU HEAR THIS.
-When you hear the Commander say this, you finish off all remaining Jackals and go to where they were. turn right, and past some small boxes that you can hide behind is a large brown dumpster against a wall. Jump on this dumpster and look out toward where the new snipers (or all the new enemies) are located.
-From here you can see a small roof with a larger one on top of it (it is a bright orange color; you cant miss it.)
-Jump on this roof and then look left. You'll see two grey pipes that are running vertically up against the next wall right next to you. Jump on these and then jump straight out onto another large roof.
-Turn right (toward those new snipers again)and go out that way. If you look up (not straight up, but look slightly up) you'll see a grey platform like thing that is very weak looking and damaged. It connects the alleyways sides together. Get ontop of this small grey platform and you can easily jump inside a building with a section cut out of it. Inside is the sword!
SERIOUSLY, IVE BEEN MAKING A BIG DEAL ABOUT THIS SWORD. ITS NOTHING COOL, ITS JUST A NORMAL PLASMA SWORD. YOU CAN EASILY GET ONE FROM CLOAKED ELITES IN THIS LEVEL TOO, BUT IT IS JUST A HIDDEN BUNGIE ITEM I GUESS...