The Hero's Path

Here's the game version I had on another computer, where I saved the complete stats and the final battle (finalniy_boy1) – I'm attaching it.

Also, on this computer, I tried to replay the battle with almost no artillery preparation (it was nice to see that I even managed to avoid any losses). As a result, the war took only 505 days (the first page of the stats for this version and the stats for the "arena" are provided last, and the final battle is called ese_final).
Conclusion: the last 100 battles in the arena were pointless, because a hundred and fifty unicorns, two thousand goblins, and azure dragons remained unused (although it didn't take much game time, or real time, to use them).

There was a version in which I managed to complete it a week earlier (I wrote about it above) – in 498 days, but unfortunately, I couldn't find the save file for that one. In that version, it's not possible to defeat the demoness with the first shot, and they manage to halve the army with their counterattack and their own attack, resulting in a very epic battle, although I was still able to win it.

P.S. I think that with knowledge of the map and its requirements, it is quite possible to reduce the completion time to one standard year, you just need to spend money wisely – hire those who are really needed and fight only with those you need to fight.

P.P.S. I was thinking about the water thing, but I just didn't want to waste time leisurely swimming down the river.