The game mechanics Google Doc for MfD can be found here.
Generally speaking, for combat, winning and losing is determined by dice rolls. It could be as a skill check or against rolls. The quest masters decided when it is time to roll, based on story events and player strategies, some of which aren't determined by dice rolls.
History[]
Mass combat was added to account for a missing mechanic during the Hidden Mountain arc, now known as Isan.
The Great Rebalancing had drastically altered some rules while buffing up every rank's stats, making it much harder for lower ranks to beat higher ranks.
However, the game mechanic wasn't adapted for large scale combat, and the QMs voted to move to a modified version of the Dresden File's RPG system, Fated to Die.
At one point, the henge technique was removed as an ability because it was too hard for the Quest Masters to plan around. When this change happened, it was referenced in-universe with an Interlude chapter featuring Kagome, who nonetheless retain memory of henge though he can no longer use it.