On the level "Halo" you have to make it from start to finish without getting into a vehicle. The easiest way I found to do this is to turn on the skull that allows you to fly. After you get the warthog, just fly through the tunnel, activate the bridge, and fly to your first dropship location. You don't have to fight the enemies in the tunnel at all. If you turn on Bandanna skull as well, you have infinite ammo and grenades making the enemies very easy to pick off without the turret.