From experience, I know how to get the skull a lot more easier.
Warning: Don't hijack the banshee yet
Step 1: Grab the "Blank" Skull from the Armory level, after you upgraded your shield then wait until Sargeant Johnson mention "Would it help if I said please?" (It should take at least 5 minutes), then go to the elevator, go to the second one and go by the window and you'll find a skull ontop of a box heading your way. Push X to grab it. This skull will not only detect you faster, but it can help to grab the banshee's attention.
Step 2: Go to the Metropolis Level... do Legendary if you want. Kill everything in your way, even teammates. Once you reached the end, kill all banshees and Wraiths except one or two (Two for co-op) when you're using the tank.
Step 3: Go at the end where you start walking inside a dark dirty looking place, make the sure banshee AI is going through the tunnel exit. If so, destroy the wings carefully.
Step 4: Next up, you need to be a good timer, If the wings are blown off, wack the banshee through the tunnel and hold X before it loads. There are two chances to lose your banshee, if you're early... you explode to death, if you're late... the banshee disappears.
Step 5: Drive the banshee out the tunnel, look up and find the 2 bridge buildings, keep going to the one the near one. It'll take a while, but you'll find a small floating Plasma Rifle in the center of that bridge, and you'll also find a warning cone right under it.
How to get the second oneStep 6: You must have a teammate with you, cooperating is the key to get the second one. You obviously just need the banshee to carry the second person ontop. Go by the area with the 3 wraiths, hidden rocket launchers at the left side with a big sign named "TTERAB" (It's Barett in backwards). Observe where the Scarab will be stationary at and place the second player ontop some sort of ledge that is able to reach the area which the Scarab shoots out.
Step 7: There are chances if the second player doesn't teleport, player one goes through the building and ontop where you'll meet somebody which will trigger the Scarab to respawn. When the Scarab shoots, the second player has to jump off and hold X to take the gun.
Now there you go, you can dual wield scarab guns as well... at least my post has the second one.
Not only, but there are other methods to get the first Scarab Gun, you can warthog launch by getting the "Black Eye" skull and the "Sputnik" skull but you'll need a teammate to spawn so you can kill them to drop their grenades for higher expectations. You drive a hog ontop of the grenades, you throw a grenade under the hog and jump ontop of the hog (Or a safer way, yet harder is to drive the hog)
You can also get the Scarab Gun in Outskirts, there are two methods (One method is completely impossible which is launching yourself), you can use one method cooperatively yet much complicated. One player must be in the ocean while the other is in the tunnel preparing to trigger the other person to teleport.