Might be easier to code a persistent Damage Taken trait that scales in relation to the number of people on it's hate list. It could be a sliding scale that is calculated proportionally against the number of players (perhaps a number of players over 8, so it only kicks in if the group is larger than the intended design), or it could simply be bracketed to kick in a boost periodically as the player numbers increase.

They are already tracking this data to calculate the rewards when it dies--the enmity tables are what determines how many seals/tomes you get. Would just be applying that same data to another mechanic during the fight, and it should be a simple enough mod to put into the damage calculation. There may already be some sort of Alpha or Delta already in use that they could just modify based on the level of this trait.