For the following few days Leonhart focused on the soldiers that were under the leadership of the Oni demon and started to attack the bases that were mostly under their control. That tactic proved to me correct as he was able to free two more fortresses in a very short amount of time. But suddenly Leonhart's efforts in this stage came to an unfortunate end.
Actually there was a reason why the Labushen fortress was built. It was used to seal a great legendary monster and the whole fortress itself was a seal. The tower obviously wanted the emperors to face more hardship by reinforcing the fortress and as Leonhart tried to skip that part the tower punished him by giving him a legendary monster over level 350 to fight.