On The Covenant, fight your way to the Scarab Tank battle and destroy both Scarabs. After that, instead of following the Arbiter into the Citadel, take a Ghost or a Warthog over the artificial cliff behind the Citadel. This can be a little difficult, due to a ridge at the bottom of the upward slope; however, you can get over the ridge by using one of the sloping ramps attached to the side of the Citadel. This will also work with the two flying vehicles (Hornet and Banshee); just let them settle on the invisible floor over the artificial cliff and get out.