There are a few mechanics when we are talking about the Demon War mini world change.
For context, there is a war waging against two types of demons: the Shaburak and the Askarak, and this war has 3 stages: stalemate, an advantage and "domination".
To advance from a stalemate, you need to have a balance of 100 demons killed from a faction in opposed to the other faction, meaning that if you kill 100 Shaburak and someone else kills a single Askarak, your balance will be 99 and the stage will remain in stalemate stage for the next server save.
If you manage to have a balance of 100~399 demons killed, the opposing faction will summon their "tier 2" monsters starting in the next server save.
To advance directly from a stalemate to "domination" stage, the balance has to be of 400 demons killed. In this case, the war will advance directly to the last stage, where the "tier 3" monsters will spawn in the highest floor of the towers. Killing more than 100 of those tier 3 monsters (for the achievement, for example) will change the stage back again to either an advantage or a stalemate position.