The Guardian Machine is an optional boss that can be fought in the Final Night of Inaba Pride Exhibit for the request, Investigate the Inaba Pride Exhibit, in order to obtain the Red Orb, the Sphere of the Altar, from the void behind it. The reward for completing the request is the Almighty Badge, an accessory which increases HP and SP by 100 and all stats by 10. The Machine Gold it drops allows Theodore to create the Curse Blade for the P3 Hero and the Setsugetsuka for the P4 Hero and Junpei, the Hammer of Repose for Shinjiro, and the Moth Fan for Yukiko, which grant a 15% chance to inflict curse, a 15% chance to inflict sleep, and a 15% chance to inflict poison, respectively.
After the request is completed, it will continue to respawn whenever the player revisits the labyrinth.
For most of the fight, Guardian Machine would be using physical attacks to hit the party. However, when "its eyes start shining", the party should defend - it will proceed to use Mind Charge and Megidolaon. Inflicting Magic Bind or Panic, while difficult, will block out this troublesome phase and allow the party to spend fewer turns on the defensive.