This is done on the level Metropolis and is easiest when done on Co-op. When you start out, cross the bridge. Take out every enemy and even your marines. When you're near the end, Cortana will say something about a "Welcome Party". Three Banshees and a few ghosts will appear. Take them all out. Then 2 Wraiths and 2 Banshees will appear. Take out both Wraiths, and only one Banshee. Have Player 1 continue through the tunnel, but have Player 2 stay outside and distract the Banshee. Once Player 1 reaches the area with the circular opening (a Jackal with a Beam rifle is present), take out the Jackal and head back to where Player 2 is.
Here is the hard part. Get the Banshee to follow both players into the tunnel. Do not board it yet! The first few times it will come at you and fly back out, but stay where you are. It should come back after you. (If it doesn't, go back outside and try again). Eventually, you'll have to get it over two road blocks. Once it is past the second one, have both players stand near the entrance of the room where Player 1 took out the Jackal. It should fly at you and get stuck. Shoot off both of its wings so it can fit thru. Have Player 1 walk slowly in front of the Banshee, making it follow him. Player 2 should be behind it, gently pushing it. Take it to the left and get ready to board it. As soon as you hit the checkpoint, the Elite in the Banshee will disappear. You only have about a second to get in before the Banshee also disappears. Both players should press X quickly. If done right, one of the players will be driving the Banshee!
Take it outside and directly up. Very high in the sky are two skyscrapers with two bridges connecting them. In the middle of the bridge closest to you, there is something that looks like a plasma rifle. It's actually a Scarab gun! It has infinite ammo, so have fun. Just don't aim at the ground.