The so-called difficulty boils down only to the number of units you need to farm to overcome the effects of Kaspar's buffs. So, I don't really see the point in focusing on it. For me, the whole point of the map is the confrontation with the demons. Once they are defeated, the player is free to humiliate Kaspar as they wish. If they want, they can do it with small forces; otherwise, they can simply overwhelm him with numbers.
And this is quite easy to fix by making Kaspar's army dependent on factors rather than being fixed. However, there are too many cities available to the player on the map, and it's not that easy to come up with something truly reliable.