From IGN How to get the banshee;
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Unlockable: Scarab Gun There is a unique gun in the level Metropolis in the single player mode of Halo 2. It is located on top of the bridge connecting two buildings after the tunnel. One method of reaching it is to jack a banshee and fly it through the tunnel. This is much easier on co-op. Another method is to use the flying sword exploit in conjunction with the marines and the warthogs. If you are on your own I would recommend shooting the wings off the banshee before you jack it so you can easily get through the tunnel. As soon as you jack it push forward even when you’re still throwing him out, so it doesn’t blow up. Once you’re out of the tunnel fly up to the first 2 buildings with a bridge connecting them. The gun is on that bridge but its kind of hard to see so fly low over the bridge. It is in the form of a plasma rifle and fires bursts similar to those that come from the wraith."
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Hint: Warthog Soccer Play a round of soccer in Halo 2! In the Metropolis level, there will come a point where you must destroy two Wraiths that are laying siege to some marines in a building (just before the sequence where you must board the Scarab). Just as you enter this part of the game you will see a few marines in a Warthog battling against a couple of Ghosts. Surrounding you will be a few building and one in particular that looks as if it has been filled up with rubble. If you can manage to get inside this building, on the lowest floor hidden in the corner is a giant Soccer ball (you will need the Banshee to get there). You can then push the soccer ball out on to the road below. If you move the witches hats to make goal posts one player, in co-op mode, can commandeer a warthog, and the other can get in another and you can have a giant game of soccer.
Submitted by Mario"
It's really hard, I don't know how to do it, so I asked. I thought there might be an easier way.
I needs one of you guys to come play halo with me! 
One day too late.
