In the "Assault On The Control Room" mission, you can get a Banshee shortly after the tank. Take the sniper rifle and rocket launcher. Kill the enemies, then go to the boulder on the right side of the tunnel. Look up to see a rock bridge. While still looking up, start moving forward until you see a platform coming out of the wall. Zoom in with the sniper rifle to the center of the platform, and then without moving, change to the launcher and fire. A banshee will fall off the ledge that you can use.