There are two ways I would do this.
For simple minions, I would use normal stat blocks and give them 1 health. 1 hit and they go down. This is for things like ninjas or low-end security guards for a NOW game. Enemies that can be a threat individually but should be easily dispatched.
For mobs and squads - groups of enemies that act as one - I would give them a flat amount of health per enemy - probably equal to their health dice pool (so a month made up of storm troopers with 5d6 health would be 5 health per trooper.) For particularly tough mobs (veteran troopers) I would double that. I would have them gain a +1d6 to attack rolls for every so many of their number - again depending on training (per 3 troopers, per 10 football hooligans). I would have the entire month gain 1 turn, but allow them to get free melee attacks against all targets within melee range. At certain size amounts I would increase the size of the mob - giving the players a bonus to hit them. AoE would get an additional 2d6 like.
This makes mobs dangerous. A football hooligan isn't dangerous alone, but get 30 of them pelting you with beer bottles and bricks or, god forbid, surrounding you, and you've got a hell of a threat. Now, disperse them with a molotov, a shotgun, or by putting enough bullets to get them to disperse and they're barely worth the time. Very forward heavy threat but with clear weaknesses, and gets weaker as it takes damage.
