Has something broke in the scenario? Because no matter how I try to attack the demon king, I can't see to push him back even while evolving all the traits that increase dmg which is a bummer

There are several ways the Demon King can be defeated. I think you got the one where a country's army finishes him off; this is can happen if the country he lands in is still well-organized and he is too tired from being chased around to fight back. The manner that he is defeated has no impact on the final ending, as long as you played at least some part in the battle.

I've found out that there is a third way to finish off the demon king (Besides the "you take him down yourself" and "the Templars use their ancestors' weapons"), but I kinda skipped passed the blurb. I remember that the blurb's icon was the one from the "Xenophobia" scenario, as well as the one you used for "The Last Man" in one of your previous works. I don't know how to trigger it again, so can you shed some light on what it is?

Video for the good ending is up.

I'm going to make a video or two to demonstrate a fight with the Demon King, since Pravus didn't make it.