Screenwriting : Tough villain by Osaze Adeyi

By trickery, a false ally lures demon into a detrimental trap eg deep lake or if no physical force will kill this demon use opposing forces of a supernatural entity. EG Ancient tree gods surround him in a forest in a magic circle he can’t escape from OR lured to a holy sight where his fire won’t work so has to do combat with an arc angel or a unicorn who locks horns with him. Maybe a “Human” protagonist needs to find a secret parable or spell which when recited will send him back to hell.

All creatures must have a weakness, that is why they introduced Kryptonite into the Superman story. You need to devise a weakness. Hide it from th characters and then let them discover it and use it.

It could be something as simple as salt water. That is why the monster lives in the mountains.

Tears of Virtue, of course you can have all manner of wicked fighting before finding that out.