How to get a Banshee sooner than the bridge
Assault on the control room: How to get a Banshee sooner than the bridge, 1 player.
Fight your way through the level to the first snowy open area, kill the enemy tank and all the rest of your enemies then hop into the Warthog. Make sure you get the Rocket Launcher and the Sniper Rifle. The next open area has a battle raging and an empty Scorpion, hop into the Scorpion and waste all the Covenant with the cannon. To the left is tunnel, to the right is the obvious way to the next open area, go to the right and clear the enemies around the corner. Stop (If you kill all the enemies in the next open area an Elite will get the Banshee before you do and use it to attack you. You'll know you're in the right place if there's another enemy tank in the area.) Now, jump out of the Scorpion and stay to the Left edge of the valley. Look up, you should see a Ledge above you with some lights on the underside. The Banshee is on that ledge so get underneath it and shoot some rockets up to knock it off. Remember to move something forward with concussion weapons you have to shoot behind it, so use you brain. If done correctly off will tumble the Banshee, if done incorrectly it will be pushed farther in. As an added bonus the last few sections of the level will have no enemies due to the fact you've skipped so much.