How did he kill them?

Farnus made a wave with his hands, and a screen revealed the training room where Xander was standing could be seen.

He wanted to know what sort of monsters Asher had fought to come out of the C Rank alive.

Farnus guessed that it was around 20 D Rank monsters, and Asher got injured while running towards the portal.

He had assumed that this Yellow Gate dungeon was a bit special, as the portal would have appeared somewhere in the dungeon, forcing them to explore and hide from the monsters. Even in his wildest thoughts, Farnus did not assume that Asher had fought all the monsters inside the dungeon.

And the fact that it was a special environment dungeon was still not known by anyone other than Asher and Alyssa.

As Asher entered the training room built for Professors, he saw Xander standing there in his formal clothing.