Mementos

Mementos is the Palace of the general public, a sprawling subway-like dungeon that grows deeper as you clear palaces throughout the game. Unlike other palaces, Mementos is available to explore at any time and features randomly generated floors. You will visit Mementos to complete Confidant side quests (Mementos Requests), grind experience and money, and eventually descend to its deepest depths for the final confrontation. Jose, a mysterious boy, appears in Mementos and trades stamps and flowers for items and gameplay bonuses.

The Holy Grail at the bottom of Mementos constantly heals itself. The key is to deal enough damage to outpace its healing. After a few turns, the Confidants you have built bonds with will rally the public, draining the Grail's power and allowing you to damage it normally. Yaldabaoth, the true final boss, uses the seven deadly sin attacks corresponding to each palace. He has multiple arms that must be destroyed. Use your strongest Almighty and Physical attacks, keep buffs active, and heal every turn. Satanael finishes the fight with a scripted Sinful Shell.