Pride and Unwillingness
Tusa was being a bit unfair to himself.
Recently ascended ace pilots generally had little chance of surviving an encounter with a greater phase lord by himself.
In fact, the Arena Lord of Ya'gwasa would have fought a lot more aggressively and put the Dark Zephyr in much greater peril if the latter had not retrieved the tier 3 Destroyer spear.
Although the nunser phase lord openly despised the weapon and how it was being used by the Dark Zephyr, the wiley old alien leader never took the Destroyer spear lightly in his actions.
His long experience had taught him better than to underestimate any weapon or opponent. Just because he did not agree with his opponent's combat approach did not mean he could defeat it with ease.
It was this habitual caution and respect that encouraged the Arena Lord to hold back his aggression and prevent himself from making any mistakes that his much more lethal opponent could exploit with devastating consequences.