There are plenty of methods to get the scarab gun. I'll be explaining two of the most popular ones below:Method 1 - Banshee
This method might seem a little intensive, but it is the one I would recommend. At the beginning of the level, kill your allies so they don't get in the way. Now continue across the bridge, and hijack a ghost. Continue to the end of the bridge where you should get a checkpoint and see a banshee enter the area above you. Your goal now is to get the banshee to follow you into the tunnel by shooting near it and reversing into the tunnel while guiding it away from the debris on the road. This may take a few tries but once the banshee follows you past the green car, you are good. Continue forward (and it will follow you), and take care of the allies in the warthog in the tunnel so they don't shoot it down. Continue to the end of the tunnel area on foot, killing enemies and keeping the banshee intact. Once you reach the end of the tunnel, get the banshee to try and follow you into the red tunnel. It won't fit because of the wings, so you will need to strategically shoot off BOTH of its wings. Once you shoot the wings off, it will fit. Make sure it follow you to correct tunnel (left one) once that happens. As soon as the banshee enters the mouth of the final red tunnel, you will need to hijack it and steer it down the tunnel! As you're hijacking it, the game will say "Loading... done". If not, you've done it wrong and are probably boarding too early. Once you board it it the tunnel, you will be able to freely fly it around. From here, fly towards the large bridge above the city. Land on the closer bridge, and the scarab gun will be floating there available for pick up.
From this location you can also take the banshee and pick up the Soccer Ball
related achievement:Method 2 - Sputnik + Rocket
** Sputnik Skull Required **
This is another method which is probably easier but takes way more trial and error, and is very hard to explain through text. You'll want to continue through the level until you regroup with marines on a bridge with a scarab that walks underneath. Pick up the rockets and head to the other
bridge (the second one the scarab gun under). What you'll need to do now is use your feet to push the small container and line it up with the H2C geometry at the end of the bridge from the side you entered. Now jump on top of this container, and wait for a checkpoint. After that, you'll have to aim down and shoot where the pipe meets the concrete piece of the bridge nearest to your location. This will launch you upwards, and if down perfectly, you will land neatly on the top of the bridge.