Goal

If you get rid of magic, everyone becomes mortal. They become weak. Getting rid of magic makes it easier to take care of “hindrances.”

True, which is why it seems this is the goal/work of a demon. Only a demon would want Serie, Frieren and other magic users to die.

But how come the Magic Special forces or Denken are not on the hitlist?

Most likely because they are connected to the royal family. The Magic Association is a completely separate entity and organization that holds no ties to the royal family which means they could very easily break their treaty and take over the royal family. The magic special forces are pretty much the police force of the kingdom, which means they are required to have magical abilities to take care of other people with magical powers, so they get a freebie. But anyone that’s affiliated with the Magic Association, especially adventurers, have no ties to the royal family so they are considered a ‘threat.’

But the Shadow Warriors are also an enemy of the Empire and Magic Special forces. It was the previous King who officially disbanded the Shadow Warriors so why would the leader of the Shadow Warriors not target the current King and the Magic Special forces? Their goal just doesn't make sense.

The Continental Magic Assoc cannot topple the Empire and take over the royal family esp when Denken is also an Imperial Mage.

Also, the Shadow Warriors didn't just target Continental Magic Assoc mages, they killed any mages even within the Empire.

One mage cannot defeat Serie. Multiple mages couldn’t even defeat Frieren’s clone in the test. That fact is what makes them want to destroy the magic association.

They also killed people, nobles within the empire. They killed everyone that was a ‘hindrance’ to the royal families ideals.