According to simulations run on different futures of Ignis and humanity, Revolver says that Lightning's is the only one that leads to mutual destruction.
The summoning condition is one 
Level 4 or lower Armatos Legio!
Die Beschwörung verlangt ein
Armatos Legio der Stufe 4 oder niedriger!
A condição de invocação é um monstro
Armatos Legio de nível 4 ou menor!