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 Nimoy Bore spawns.
Strategy 2 (feat. Gigavolta and Aphrodite)
This lineup is overkill. Spawn Legs to kill Celeboodle, use the money to spawn Gigavolta and Megaphrodite. Easy win.