Arcturus also had a much easier time when he finally started fighting with the Dimension Empyrean because it felt far more natural and innate for him to fight with the Dimension Empyrean than with the Psionic Abilities.
He was trained rather extensively by the Grandmaster Byrmellir in combat using the Dimension Empyrean, and become he had very similar Super-High Risk Primal Psyker Ability; his master was able to give him many useful tips when it came to the usage of the Dimension Empyrean.
It was evident that the Headmaster of the Imperial Phoenix Military Academy wasn't going to actively fight because that would be an absolute squeal fight, as he was already Beyonder, so Arcturus was prepared that the Headmaster Kane would be only dodging his efforts to hit him.