Aragorn was still in the process of digesting his gains from his evolution to the Advanced mystic realm, while Rex was still in the process of understanding his new sword and fully mastering how to use it.
Berserker's new form provided a lot of versatility to his sword style but it also increased the skill level for Rex to make perfect use of it.
For him to be able to use the sword as fluently as before and possibly exceed his previous limits, he needed practice and Rex fully recognized this fact.
Also, Rex intended to integrate his swordsmanship with his morphing forms to fully complement his Bear-man form.
Even in his Half-bat form, Rex never really gave much attention to his swordsmanship. Against this powerful enemy though, he felt that any power-up was necessary and mastering his sword in his different forms mattered.
