No, there’s no legitimate way to defeat Duma in the final battle. Though, for some weird reason his stats do lower if you reset and reload a suspend point, but it should still be reasonable impossible. It’s no intended one way or another. The last chapter is basically an interactive cutscene. I did intend to do some post game trial maps however, fighting the various famous character from it, but I could never figure out a way to restrict and return Archeon’s movement in a way that would actually let you use his promoted class in trial maps but also functionally isolate him for the final map.
Very happy you enjoyed it, I don’t think many people have managed to complete it! I think I made the difficulty a tad bit unreasonable. But hey, massive difficulty does suit the theme of the plot. Since you enjoyed it, you might be interested in my current project in development. Pyrathi 609 (though if you liked Fall of Thabes sky high difficulty, you might be disappointed as this game is significantly easier).
https://feuniverse.us/t/the-pyrathi-project-recruitment-drive/18515
Also set in the Valentia-Archanea universe, but sequel to Mystery of the Emblem, rather than a distant prequel. So far about 7 chapters are available, but I plan on dropping an update soon making up to chapter 13 available. I have been a bit distracted with Engage for the last month, however.
Anyway, really happy you enjoyed the game. I put a tonne of work into it, glad to see some people are playing it.