The main focus of the Holy City's attack on the Mechanical City was on the surprise attack. Even though the number of mechas the Holy City had was almost several times that of the Mechanical City, the Bishop of the Holy City wanted to resolve the war in a short period of time. He did not want to be dragged into a fight by the Mechanical City.
This was because there was a faction south of the Holy City that looked very weak, but could not be underestimated. It was an area controlled by the human Federation.
In fact, this was why the Virgin Mary of the Mechanical City was even more worried. After all, New York Base City was not far from the Mechanical City. Most importantly, the Virgin Mary was the only person who had fought with the God of War Peter, who was from the human camp.