Most Ancient is the fifth stage in Spring, Sprang, Sprung.
The Face appears as a boss at the start of the battle.
Easter Bunny and St. Pigge the 2nd appear as main threats.
B.Bunny and eventually Sir Seal appear as support.
After 100 seconds, a Shy Boy will appear.
An Awakened Bahamut can help you rush the base down fast. Additionally, Swimmer Cat can help to deal fast damage to The Face that pops out as a boss at the start of the battle.