Basing how many guardians we have off of what was posted in the bungie update we could split the guardians into 3 different groups with literally millions in each group. One group would be used as builders along with civilians to use their ghosts to turn useless junk from outside and inside the city into parts or pieces (since the ghosts have the ability to repurpose material from when he gives you armor). They would take these parts and would be used to start building anti air armaments, vehicular shuttles, and outpost defenses that would then be brought out to the fringes of the city so after the areas around have been cleared can be moved out to create defendable outposts not only from ground troops but from aerial attacks too. This group in tandem with the group of guardians tasked with eliminating all life outside of the city would be able to start spreading out more and more while the third group is able to stay behind guarding the city from an chances of direct retaliation. That's the one thing to me that seems to be why they never gain any ground back is that there's almost no protection from enemy ships that come into the atmosphere.