Sparring for several hours in the most intense conditions was something that was rather tiring, yet at the same time, it was rather beneficial for him. Arcturus rarely had the opportunity to train with the Dimension Empyrean under such high duress but in rather a safe environment and conditions.
It was something unprecedented because normally, when he was using the Dimension Emyprean, he was using the Super-High Risk Primal Psyker Ability only when he was fighting against the enemy that he wanted to kill.
Due to the destructive and sometimes even unpredictable nature of the Dimension Empyrean, Arcturus was refraining from using the Super-High Risk Primal Psyker Ability in the sparring matches, as such a thing could be pretty dangerous.