Most people think that the monster comes at a certain time, and cannot be changed. But the truth is that we can choose when we want it to be there.
First of all, we must differentiate that the monster in a small city is not the same as the monster that is activated in a city with a large experience level. In the latter, the time the monster spends in the city is less: 6 hours. In a level 18 city, however, the monster's time is 12 hours.
For this trick, the only thing we have to do is to control when the arrival of the monster to the city is stipulated, and not to get into the game until we want to activate the monster.
With an example we can see it better: let's imagine that the monster comes at 0.00h at night, and we had planned to sleep at 0.30h. If we wait for it to arrive we will only enjoy the advantages of having the monster for 30 minutes. However, if a few minutes before the arrival time of the monster, we close the game, until we do not open it again, the monster is not activated, so when we open the game in the morning, it will be when those monster hours start to count.
The main advantage of having a monster in our city is not to fight against it, but to activate the shipments and we can complete them to get battle items. That is why it is so important that the monster stays as long as possible in a place where we can take advantage of its benefits. It is of no use if the monster arrives in the wee hours of the morning.
Important notes: The game must be closed at least 2 minutes before its arrival. This is because, sometimes, the game takes 1 minute to close, and remains dormant in the background, so the monster could be activated if we do not close the game correctly.