Mammon Machine will launch a powerful AoE once it releases energy. Generally you want to kill it before it casts the AoE. Attacking with physical-based attacks will raise its physical defense. Attacking with magic will increase its attack power. Attacking with Frog using the Grand Leon will reset the Mammon Machine's defenses. If your party survives the AoE, Mammon Machine's defense will remain unaltered until it begins collecting energy again.