I play by my own rules ... But i love a game of snipers!!! :)
Here is the complete walkthrough to getting the scarab gun, i believe it is the best thing in all the halo games, please bump so more people can finally get the scarab gun for themselves!
Method 1
Note: The Sputnik Skull is recommended for this, but not necessarily required.
At the beginning of the Halo 2 level, Metropolis (Easy difficulty is the only practical way to attempt it), kill all of your allies. It is not necessary to kill the enemies on the bridge. Simply driving past them is sufficient to obtain the Scarab gun, but if you are playing on a higher difficulty, you may have to do so. At the point where the two extra allies show up on the bridge, the first two Banshees show up. Saving as many Banshees as possible will not help later as they tend to disappear once you enter the tunnel. The only practical way to have a backup in case you fail is to lure a Banshee into the tunnel, get a checkpoint and have the Banshee stuck in some convenient place (such as between two trucks/cars). Be sure to kill the marines that come on the Warthog. Next, clear the tunnel ahead. Stop, once you reach the closed section of the roadway and have to detour off into the pipes. (Make sure to kill the sniper in there before heading back.) This will leave the tunnel enemy free, making it easier to work a Banshee down the tunnel to the spot needed. This is where the Sputnik Skull may come in handy.
Because your melees make objects move further, it will be easier to maneuver the Banshee where you need it. Return to the end of the bridge and move a tank to road on the bridge. Land a Banshee on top of the tank and then drive the tank down the tunnel. When you reach the first road block, you will need to position the Banshee on the top of the ramp to the right. It is important that you don't attempt to fly the Banshee once in the tunnel. It will cause you to die. You are not able to fly the Banshee into the tunnel due to an invisible wall. Now by hitting the Banshee you can maneuver it through the walk way at the top of the ramp and past the road block. To get the Banshee through the walk ways and end tunnel it may be necessary to shoot off the wings. To do this use either a Plasma Rifle,Battle Rifle or a Sniper Rifle and shoot single shots until the wings are deformed. Don't hit the Banshee too many times in succession. Allow it to rest between attacks, or it will blow it up. Two melees is the most that the Banshees can usually take without needing to rest.
Work the Banshee past the second road block, by forcing it up the wall to the right and through the walk way again. Force the Banshee into the tunnel on the right and up the first ramp in the tunnel. You need to position the Banshee (once on the platform at the top of the ramp) so that it is facing into the next tunnel. Your Banshee should be upright and you should be able to walk up to it and see the "Hold X to pilot Banshee" message. Now force the Banshee, into the tunnel by using well placed and strategic hits. Once the "Loading ..." message appears you need to enter the Banshee before it disappears, If you don't make it you will have to revert to last check point and clear the tunnel again. If the Banshee is destroyed, it is necessary to go back and retrieve another and force it down the tunnel the same way.
After "Loading...done" disappears, fly through the tunnel and then directly up and to the left. There will be two suspension bridges; head to the first one. On the middle of the suspension bridge is a cone over which the Scarab Gun is hovering.
Method 2
Requirements: Lots of patience.
First, kill all the Marines and eliminate everything but one Banshee and one Wraith at the section before the tunnel. Before taking the wraith, use the scorpion turret to shoot off the wings of the last Banshee. Do not shoot too many times in a row or it will explode.Steal the Wraith, luring the Banshee through the tunnel. The reason why you need a Wraith is to better protect yourself from the incoming super heated plasma bombs from the Banshee (the Banshee WILL shoot at you) and to easily kill your allies in a Warthog later in the tunnel. While heading down the tunnel, do not race too far ahead, as a checkpoint will be triggered and the Banshee will disappear. On a similar note, do not let the Banshee draw too close before getting to the lower part of the tunnel, because it will turn around and leave. Try to keep the Banshee within track of your radar.
Kill all enemies in your way as you continue. Due to the AI wanting to do complex maneuvers, such as turning around or turning in general, it will occasionally get stuck in a crack. To fix this simply go directly behind it and move backwards until it turns around and gets itself out of the crack. Or, if it gets stuck near your enemies, you might want to leave it for awhile and kill the enemies first, thereafter returning to free it. This will ensure that it doesn't fly away while you're not keeping a keen eye on it. However, while doing this, you must make sure there is not too great a distance between you and the Banshee or it will disappear. Eventually, when you and it are in the service tunnel, lure it to the point where the loading screen appears. You have to board it within one second. Now fly directly up to a suspension bridge found in between two buildings (note that there are two bridges, go to the first one). Go to the middle of it and claim the gun.
Method 3
Requirements: Sputnik Skull, Black Eye Skull and an Overshield (Using Black Eye Skull).
First of all get the Black Eye Skull to get your shield to overshield range. Then drive a Warthog to the command outpost, and up the small bridge at the farthest from the outpost. Be sure your Rocket Launcher is either your primary or secondary weapon. Stand on top of the gunner part. Fire the rocket at the bridge. This is when the Sputnik Skull comes in handy, however, there is a high probability of death. Once on the bridge walk to the Scarab Gun and take it. Try it on multiplayer and hop off the bridge, die, respawn with another player and get your gun back.
Method 4
Requirements: Sputnik Skull and another player (strongly recommended).
First of all, get a Rocket Launcher and a Battle Rifle. Then, continue until you get to the part where you have to destroy the Scarab (The last part of the level) and get as much ammo for your Battle Rifle and Rocket Launcher but not necessarily required. Then, zoom in your Battle Rifle at the last bridge where there is no shelter and the connector to the second building. Shoot the box beside the middle exploding vent.
Once you shoot it three times, zoom out and head to it. Push the box and align it with the drain at the start of the bridge. Make it as perfect as possible, then jump on top of it. It is strongly recommended you wait for a checkpoint so you don't have to repeat the entire procedure all over again. Once the Checkpoint...Done appears, switch to your Rocket Launcher and shoot a rocket near the vent nearest to you. You will then fly up towards the bridge where the Scarab Gun is located. This takes some tries. Sometimes, you will go too low, sometimes, you will hit the bottom of the bridge and mostly, you might go too high and hit the invisible ceiling and fall to your death.
But after a few tries, you will be able to get it. This is far most the easiest way to retrieve it. It is suggested you do this in Co-Op mode because when you retrieve the deadly weapon, you will not be able to get down. In co-operative, you can position the second player directly under the suspension bridge. Then, drop to your death and the second player would most likely retrieve it.
PS: ALL credit to the halo wiki! Please bump