Ancient Trial is the third stage in Sub-chapter 26, Sea Polluter.
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 the Nimoy Bore spawns and win the level.