Hello!
I'm looking for a way to fight more enemies to train, gather experience and so on.
Can I somehow set back an already entered area to fight the monsters again? Or is there any kind of respawn mod? I was looking for this in the whole web and there seem to be many people scrabbling about something like that in many forums but I could never find a way. Any expert here who can help me?
Thanks a lot! 
Hiyas Kicker

Afraid that I have bad news for you on this one.

There isn't any way to reset an area that you've visited if it's in your save game. And the game itself doesn't use spawners, so there's no way to get them to reappear either.
Now, I do know that there was a mod from a while back that installed a new adventure to the game, and had resettable spawners. But unfortunately, I cannot for the life of me remember what it was called

For more adventures, you would need to either buy the DLC packs to install new adventures, install mods which add more adventures, and/or travel more, so that you are getting more fights between cities.