Vessels are the damage gate
The Ancients have 60 base HP, 2 DEF, Flying and Double Turns. Call for Help produces two Vessels. If enemy slots are full, it becomes Restoration, healing the enemy team by 4. Clear the Vessels rather than leaving them alive to avoid another summon.
Follow the six-move loop
Wheel → Beam → Call for Help / Restoration → Fireball → Ultra Beam → Darkness. Double Turns means two moves can happen during one enemy turn; a support animation does not guarantee the rest of the turn is safe.
Prepare before Ultra Beam
The closed eye and gathering orange energy precede 19 piercing laser hits. If you cannot guard that sequence, Ghost Potion is an available avoidance action at 1 SP, followed by Exhausted. Arrange recovery before spending that action. Darkness then hides enemy HP and status information for two turns; continue tracking the last visible states instead of treating missing bars as a reset.
Mechanics references: Ghost Potion
Where the fight fits
This is the Chapter 4 finale at Mount Red Sun, following Captain Trotter. Answer Guilty to Guru for its Nirvana rematch.