Play on Easy and activate the Sugar Cookies, Pain Train, Bountiful Harvest and Emperor Skulls. Set a Global Rally Point beside the Power Core and continually send Cyclopes to repair it. Use the Heal and Repair power on the core and send Spartans to hijack enemy vehicles to save resources.
Play on Normal difficulty and move Forge as well as the 2 Marine squads to the middle of the ship with the Spartans and the Hawks. Send the Wolverines to the Power Core to prevent Spirits from deploying troops. Spend all your resources in creating Cyclops and using the Heal and Repair power. Let the Spartans hijack Locusts. You shouldn't have to create any more Marines squads. As of the third minute of the game, you should have at least 10 Cyclopes, and barely any Covenant able to bypass your defenses.