That clears up a lot.  So basically, our characters would be from Earth in the year 2025 one moment, and then something would happen, and then they would find themselves in Mer with a bunch of new skills, superhuman capabilities, and supernatural abilities.  I guess they would also arrive with some (or all?) of their equipment?

Now for a few more questions.  How realistic do you want to be regarding logistics?  Like, if my character is proficient with M4 Carbines, will their M4 Carbine become useless as soon as they expend all of the mags they arrived at Mer with?  Would the people at Mer somehow have the ability to fabricate 5.56x45mm cartridges?

What kind of equipment would the demons be using?  Would they mostly be using futuristic or modern-ish stuff, or would the demons be fighting with claws, flaming swords, and fire breath?

I guess the idea of using all kinds of weapons is cool, but there is a reason that swords have mostly been phased out of combat.  I would think that being a master duelist wouldn't help my character that much if they found themselves having to clear machine gun nests or defeat tanks, battle mechs, drones, planes, helicopters, etc.  If the demons are using sufficiently advanced technology, I can see abilities like turning into a big golden dragon becoming disadvantagous, since you would then become a large target that could be subjected to a bunch of shells, large caliber bullets, and missiles.

On a related note, what kind of punishment are you expecting our characters to be able to take?  Would our characters be able take a dozen shots from a heavy machine gun and survive, or get shot in the chest by a demon sniper from half a mile away and not have to spend the next few weeks at a hospital?  Would our characters have armor that could defeat several hits from the enemies we are facing?