Strategy: A Crystal in the Lunar Subterranean knows how to beat it!
Spoiler: Use physical attacks with non-elemental weapons, otherwise it nullifies all damages. Most effective with blunt weapons, such as Hammers and Morning Star, as they deal 4x damage vs machines. Certain actions, such as spells and summons, will trigger deadly reaction so watch out!
Optimum:
Cecil - Morning Star
Kain - Wooden Hammer
Edge - Throw
Rosa - Heal