Welp, I thought Vampire Lords were absurdly strong, but then I discovered the top level Named Beasts. I tried a game using them as an addition to my forces, but discovered that they are annoying to control as they tend to wander the wilderness. However, I also observed that they are insanely strong; first level creatures with stats like 45 attack and 35 defence!
So I tried an experiment. I didn't build a "real" dungeon, I just made a storage area, library and treasury built completely open as fast as possible while rushing for the high level beasts. I then explored my way to the village and castle, placing cages outside the castle entrance and near the elf village. Finally, once I had enough mana, I began bringing in Named Beasts. The result?
Well, it was the most hilariously easy and one sided fight ever. The first beast strolled into the castle, easily murdered the gate guards, and generated an absurd amount of mana for me. After this, all I really had to do was create more beasts, who quickly killed a bunch of people, generating more mana to get more beasts. The Duke and Elf Lord were capable of killing them, but that was with a fair amount of backup, so once the beasts started to overrun them they died pretty fast. I didn't even need to do anything.
I tried various variations such as training up my keeper in magic, equipping him with black market potions, and sending him directly into battle while looting bodies to increase his stats. In the process, I somehow lit the forest near the Elf village on fire and killed about ten of them inadvertently, including the Elf Lord. Anyhow, it became quickly apparent that Beast Rushing is easily the fastest, most efficient and easiest way to win the game, even beating out Vampire Lord Spamming. I actually wiped out the castle without them even attacking once, and most other times I beat the castle and village after maybe one or two attacks (very hilarious attacks, in which the "attackers" were massacred at their own front gates).
On the other hand, how do the skills in Adventure Mode work? So far as I can tell I don't level up, and while I can become exceptionally powerful through high level items and enchanting items, I suspect that will only go so far (for reference, the best I've done is reach the 5th level of the dungeon with about 33 defense and 25 attack before being eaten by a kraken).
Also, this might possibly be a glitch; stealing from the shopkeeper results in the entire town coming after you, but brutally murdering him beforehand will let you get away with it scot free. Even better, the optimal way to murder him is to hit him with his own sleep potion before stabbing him to death, so you can use stolen items to rob him (he only has one health potion and his stats are about equal to yours, so if you have the jump on him you should have an easy time killing him). This provides fairly substantial boosts in early game.