Your main objective is to simply repair the Power Core. Send the nearby units, consisting of a Cyclops and John Forge, as well as some backup to the Power Core. You'll want to secure the Power Core, so have a Hawk cover the repair units, and create more Cyclops in order to speed up the process. Also create a few Marines and Hawks to deal with any incoming waves of Covenant. Be sure to build four turrets in the center of the ship and upgrade them with the Flamethrower upgrade; This will help against the Covenant infantry that will occasionally move out to the Power Core.