I lost all but one of the ships and you can still win with only one by the way.
So, you could just try to fortify one…I forgot how I did it exactly…but I think I just kept mass producing marine units and covering strategic locations with them, and keeping spartans and some marines right near each ship. (one spartan and a couple groups of marines). dont forget to put marines in the sniper towers.
if im online and you want me to help you just shoot me an invite.