Apparently, Ladies Like Rail Shooters

Once you've made it through the underground highway tunnel at the end of Outskirts, you might think you're through with long, linear environments. Not so.

As you emerge from the tunnel opening into the daylight, the Sarge greets you with a present: a Scorpion tank. "Oh," he grins, "I know what the ladies like." as he gets behind the chain gun of a Pelican, a weapon so neat that we never get to use it ourselves.

The Sword Flying trick was first extensively demonstrated on Metropolis. The procedure for executing it is somewhat complicated; it involves parking a Warthog in an area where a Phantom attacks, targeting the plasma sword on one of the Phantom's turrets, and then waiting until the Phantom has withdrawn before exiting the vehicle. You will be propelled in whatever direction you face; this method can be used to extensively explore the level, including otherwise unreachable areas and is a way to retrieve the Scarab Gun without a Banshee (although you can't get back down with it...)

Now, this trick is 2 in 1. This should probably be attempted at the Easy level of difficulty. Grab a Warthog at the beggining of the level. Drive to the end of the bridge and you will see a tunnel. Drive down the ramp backwards so the Banshees flying around outside notice you and come in after you. When you get to the first half closed door, kill your marines so they dont shoot down the Banshee. Get the banshee to follow you through the big yellow doors then kill all the ground units in that section.

Later in Metropolis, Cortana says there some marines in a building who need help; she wants you to go into the building where there is a gunner on the roof and the Scarab is coming straight toward you and shoots a Scorpion. Instead, don't go in; grab a Warthog and go up the second ramp, the one closest to the entrance where some marines are standing. Park it and get on the roof. In a short time , you'll see a Pelican zooming in and stop right above you. Jump up and press Xand you'll board the craft.

The Metropolis level has a lot of cool stuff in it, but it's also a bit tricky. Get to the middle of the bridge where there is a hole in it that reveals the tunnel below. As soon as you get a checkpoint, get out and kill any passengers riding on your Scorpion, because if you don't they'll probably shoot down the Banshee you need. Kill one of the banshees but leave the other.