It is worth 5 Gamerpoints.
Play on Easy and activate the Sugar Cookies, Pain Train, Bountiful Harvest and Emperor skulls.
Because of the limited number of base sites you'll have, use every base site saving method available, i.e. dismantle the Field Armory when you're done with it, fully upgrade infantry at the Barracks then destroy it in place of another Elephant, prefer upgrading Reactors instead of building more.
Make Vultures and destroy every Covenant Base. Bring the Elephant up the ramps to end the level.
Simply don't purchase another base. Very simple.