Method 1 - Banshee (From Scarab Gun)
Follow the direction to the scarab gun using the banshee method below:
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 the scarab gun location, you can take the banshee and fly downwards towards the damaged building closer to ground level. Inside this damaged building you'll find the soccer ball on the ground floor.
Doing this method will also unlock the Scarab Gun related achievement:Method 2 - Sputnik + Grenade Jump
** Shown at the end of the above video **
With Sputnik enabled, continue through the level until you come into an area with a large and tall glass pane window in front of you. The window will be cracked about halfway up. For this method, jump on this window's ledge, and use a frag grenade to "grenade jump" up and over the glass and land inside the building. This may take several tries, so reload your last checkpoint if necessary. Once you land inside the building, you'll have to use the damaged support beams to get on top of this building. Once on top of this building, you'll have to jump over a small gap to get into a damaged building with a huge hole blown in the side of it. Once you get there, you'll see a soccer ball on the ground floor!