Okay, the game is finally getting quite challenging at around level 25/26. I am finding that there are lots of enemies listed as Overwhelming or Impossible. This is with 19,000 leadership. I haven't been able to fully clear out the Land of the Dead or Demonis as yet for this reason, even though I have completed the main quests there. It's actually a bit sad that the main quest fights are easier than the ordinary ones.
In Haas' Labyrinth, the 7 dragons are listed as Impossible. I just beat one fairly easily though using my 'all elf' team (Hunters, Elves, Lake Fairies, Sprites, Dryads). Unfortunately, there are no hordes of Lake Fairies or Dryads so I am having to use Sacrifice in the Gremlin castle to replenish my stacks. This is going to be quite boring and time consuming. I spend a lot of time waiting for mana to regenerate...
It would be better if I had the dwarven telescope for the archers and the elf wife, but no matter. One of Haas' dragons had two stacks of 360 Knights each.
But they were easily killed with slow/Sprites etc. I had wanted to finish the game in around 30 days but all the stuffing around trying to clear out Demonis and the Land of the Dead has meant it's day 34 already.
Oh and I have 2.2 million gold, so it's just a matter of time....