Play metropolis untill you get to the part with the scarab. When it cant walk any farther, board it. DO NOT kill the enemies, or this is pointless. Jump on the top (the catwalk thing thats above everything) and jump straight into the turret (scarab cannon). move all the way to the left side (if facing cabin (inside of scarab)) or right side if facing the way the cannon fires. At the end, you should be able to see insdie the cabin, and fire in it, while the covenant cant fire at you.