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The factors that decide the outcome of a battle are many, such as the means of attack, means of defense, movement speed, agility, and the sustainability of True Yuan energy, among others.
However, if one were to pick the "most critical" among them, Yan Yu would choose the sustainability of True Yuan energy without hesitation.
The reason is simple: once the True Yuan is exhausted, no matter how fierce your offensive means or how thorough your defenses, no matter how exaggerated your speed and agility, without the support of True Yuan, all of these capabilities cannot be utilized, and one would become a fish on the chopping board, ready to be slaughtered.
In his previous life, what Yan Yu studied the most was how to plan the limited external True Yuan within a finite time to achieve as many tactical objectives as possible—this was also the initial reason he dared to let his teammates withdraw and faced the Demon Monk alone.