@wanfai
This happend in chapter 54
it was one of the first tests( 2nd i think) he had a combat trial where the more monsters he killed the more points he got.
he basically tortured her because he got points for every head he "killed" but her being a hydra just regrows them.
so he farmed her for points for like a really long time which made her pretty traumatized.
The funny part was she was supposedly the "strongest monster" they were able to deploy for that challange if not in the entire tower, but that's unlikely.