Load up rally point Charlie. In the room where you have to protect the Pelican from waves of Covenant, you can board its turret. Crouch at the side of the ship near the nose, just behind where it meets the turret. Hold
(or whatever button you use to enter vehicles depending on your controller setup) as you crawl underneath it and you will climb into the turret. Just rotate the camera and fire until it stops blocking your view then use it to melt the enemies if you wish.