Strategy 1 (all-stars)
As long as you have Awakened Bahamut and very strong anti-alien treasures, this stage will be a complete breeze.
When Galactic Overseer Nyandam spawns, use Macho Legs to kill a few enemies. After you get enough money, summon Awakened Bahamut. He should quickly kill Nyandam, and you can destroy the base even before Nimoy Bore spawns.